NEW EDITION
OF
HINTS
ON
Silvered-Glass Reflecting
TELESCOPES

MANUFACTURED BY

MR. G. CALVER, F.R.A.S.

WITH DIRECTIONS FOR SILVERING, ADJUSTING, &c.

1877.

GEORGE CALVER,

HILL HOUSE, WIDFORD,

CHELMSFORD, ESSEX.

HINTS
ON
Silvered-Glass Reflecting Telescopes.

Of the various forms of Telescopes now in use, each has its own peculiar advantages; but the Silvered-Glass Reflector is undoubtedly gaining favour among our practical observers. A well-figured speculum, being perfectly free from chromatic aberration, gives, in a proper condition of the atmosphere, the finest possible definition of the Moon and planets, the markings and colours of these objects being excellently seen; while coloured stars, such as Albireo (β Cygni), or Almaach (γ Andromedæ), are exceedingly well shown, the beautiful contrast of the stars in the former being especially noticeable in a reflector. The advice of “F.R.A.S.” (in the “English Mechanic,” March 21st, 1873) as to the choice of a Telescope, may here appropriately be quoted. After expressing a preference for refractors when measuring close double-stars, he says, “But should the object of your correspondent be merely to regard the wonders and beauties of the Heavens, or notably, should he purpose to devote himself to the study of the physical structure of the Moon and planets, then by all means let him obtain the largest reflector he can afford; its absolute achromatism tells most astonishingly on these last-named objects.” This is the opinion of one who has great practical knowledge of the different forms of Telescopes.

If Achromatic Telescopes of large aperture could be made as cheaply as reflectors, and in as convenient a form, they would doubtless be preferred for general star-work, although the aberrations, especially that of colour, cannot be so perfectly corrected. A silvered-glass reflector is, however, much cheaper than a refractor, and, when the aperture exceeds five or six inches, is much handier to work, and occupies less space, being only about half the length of an achromatic of the same aperture. It is true that a reflector will give less light than an achromatic of equal aperture—but this is, in certain conditions of the atmosphere, a distinct advantage, the extra aperture to give the same light adding to the definition and penetrating power. An example of this is seen in the beautiful definition given by an unsilvered mirror on brilliant objects, as the Sun, Moon, and Venus. In large achromatics, the distressing excess of light has often to be reduced by diminishing the aperture or using a higher power than is convenient; and in such cases a lower and more suitable power can be employed with a reflector.

When the air is unsteady, the definition of Reflectors, owing to their tubes being open, is more liable to fluctuate than that of refractors, although when a reflector does not give good definition on account of the atmosphere, a refractor of the same aperture will certainly not perform satisfactorily. Sir John Herschel has shown that when the air in the tube of even an achromatic is disturbed in turning from one object to another, good definition does not return until it is brought to rest. In order to reduce the vibration of the air in the open tube, and also that of the stand, to a minimum, reflectors require to be very firmly and steadily mounted, and to have iron tubes.

Many of the specula made by me are now in observatories, where they have been compared with achromatics of first-rate quality with the most satisfactory results. For instance, several 6-1/2 inch specula were tried with two excellent achromatics of 4-1/2 and 6 inches aperture, when the planetary definition was considered to be superior with the reflectors; and the appearance of the star-discs, with equal apertures, differed little from the beauty and hardness of the images given by the achromatics. In viewing stars of great altitude, the use of the refractor is extremely inconvenient to the observer, whose position is necessarily very uncomfortable, while with the Newtonian reflector any part of the heavens may be observed with the same ease and comfort. In short, a good speculum, well mounted and situated, is sure to be both pleasing and satisfactory.

It is perhaps unnecessary to remind the reader that, when the defining powers of a telescope are put to the test, as much depends on the acuteness of the observer’s eye and the practice he has had, as upon the perfection of the instrument and the fineness of air. It is a mistake to suppose that when the stars appear to be the brightest to the unaided eye, definition will be at its best, though this may happen; when it does so, the astronomer should make the most possible use of the opportunity, as such nights are very scarce. As a rule, the best nights are those when there is the slightest possible mistiness of the atmosphere, and for the faintest stars absence of bright moonlight. The 5 inch mirror is guaranteed to divide stars one second apart with ease, and closer ones in very fine air. The 8-1/2 will split such difficult pairs as γ² Andromedæ, and μ² Boötis. An acute eye will master these stars with even a less aperture on very favourable occasions, γ² Andromedæ having been seen with a 7-inch stop of a 10-inch mirror, and Mr. Sadler, of Honiton Rectory, has split this star with his 6-1/2-inch telescope. It sometimes happens on favourable nights that the most difficult objects will be seen with the same telescope, which on other occasions had failed to show them as well as a much smaller aperture had done in very fine air. These remarks equally apply to the observation of minute stars and planetary detail. The amateur must follow the advice of the Rev. T. W. Webb, given in “Celestial Objects” (pages 15-17), and must cultivate that virtue applicable to all scientific investigation, namely, patience. The following interesting and difficult objects may be looked for, the powers used for their observation should vary from 150 to 300, and for the very closest stars up to 500, or even still higher.

TESTS FOR SPECULA.

Light Tests.

94 Ophiuchi. 5″ : 7, 13.

58 Ceti. 3″·5 : 6·5, 14.

γ Crateris. 3″ : 4, 14.

15 Monocerotis. 25″, 15″ : 6, 9·5, 15.

τ Orionis. 15″, 20″ : 4, 15, 12.

υ Ceti. 6″ : 4·5, 15.

ε Trianguli. 5″ : 5·5, 15.

179 Piscium. 3″ : 8·5, 15.

110 Herculis. 55″ : 5, 16.

μ Andromedæ. 45″ : 4, 16.

β Equulei. 35″, 3″, 50″ : 5, 13, 16, 14.

85 Virginis. 30″ : 6, 16.

55 Andromedæ. 25″ : 5·5, 16.

178^a Delphini. 20″ : 7·5, 16.

212 Libræ. 20″, 10″ : 6, 16, 8.

14 Monocerotis. 10″ : 6, 16.

94 Ceti. 5″ : 5·5, 16.

δ Aquilæ. 1″·5 : 3·5, 16.

Separating Tests.

33 Orionis. 2″ : 6, 8.

52 Orionis. 1″·8 : 6, 8.

δ Cygni. 1″·8 : 3·5, 9.

2 Camelopardi. 1″·7 : 5·5, 7·5.

π Aquilæ. 1″·7 : 6, 7.

σ² Cancri. 1″·4 : 5·5, 7.

9 Leonis. 1″·2 : 7·5, 7·5.

η Orionis. 1″ : 4, 5.

ε Arietis. 1″ : 5, 6·5.

4 Lyncis. 1″ : 6, 7·5.

37 Pegasi. 0″·8 : 6, 7·5.

749ξ Tauri. 0″·8 : 7·1, 7·2.

46 Arietis. 0″·8 : 8, 9.

λ^a Cygni. 0″·7: 5, 6.

β Delphini. 0″·7 : 5, 5·5.

20 Draconis. 0″·7 : 7, 7·5.

287 Draconis. 0″·7 : 7, 8.

178^b Delphini. 0″·6 : 4·5, 6.

φ Draconis. 0″·6 : 4·5, 6·5.

γ² Andromedæ. 0″·6 : 8·5, 9.

μ² Boötis. 0″·5 : 8, 8·5.

ι^a Boötis. 0″·5 : 4·5, 4·5.

7 Tauri. 0″·5 : 6, 6·5.

108 Draconis. 0″·5 : 9, 9.

η Herculis. 0″·3 : 3, 8.

42 Comæ Berenices. 0″·3 : 4·5, 5.

ω Leonis. 0″·3 : 6·5, 7·5.

Since writing the first edition of my catalogue, the writer has received many gratifying accounts of the success of the “Silvered-Glass Reflector.” Many private letters have been received by him from observers, expressing their delight and satisfaction. This has been so encouraging that no pains have been spared, nor any opportunities neglected, in turning to the best purpose every valuable lesson that continued practice and experience may have suggested in the manipulation of specula from time to time, in order to secure the best means for obtaining the highest excellence of defining power.

Many modifications and improvements have been made in working and testing mirrors, and special machinery and appliances constructed for large sizes. But to complete my conditions suitable for truly figuring and testing specula of considerable size, I found it necessary to remove from the traffic and tremor of a town to a still and tranquil situation in the country.

The truth of the curve is so sensibly and seriously affected by vibration constantly going on in and near a town, that it is liable to a variety of defects, and the surface becomes wavy and “plucked.”

As an instance, I may mention that my first and most convincing proof of the advantage of the stiller situation was tested by an 18-inch speculum (on which much labour had been bestowed); it was laid aside, but successfully finished after removal, and without undulations or any perceptible defect, and the Observer wrote me, that in good air, he “saw Sirius as a brilliant white dot, without a ray or appendage of any kind, and celestial photographs obtained with it are very fine.” Such results were exceedingly gratifying, as they were obtained with much less labour and uncertainly, and the tedious process of the final touches had not to be repeated so many times.

It is said that the celebrated Alvan Clarke, from the same effects of tremor, never finished an object-glass to his satisfaction above-ground; and Dr. Draper, testing his mirrors at the centre of curvature, to avoid draughts, &c. in an ordinary apartment, resorts to an underground one.

In this little book of “Hints” it may be useful to remind those who possess a speculum of fine quality that they are not produced by a “rule of thumb”—so to speak—and that the difference between a speculum and a really fine one, giving a maximum of defining and illuminating power, is the result of considerable labour and thought, and deserves careful usage.

The Rev. Cooper Key, an amateur of much experience, writes in the “English Mechanic,” that he was eight months (working sometimes eight hours a day), giving his 18 inch speculum its final touches and corrections.

A well known correspondent of the “English Mechanic,” “Hyperion,” tells us he found it impossible to test his 8-1/2 inch mirror in an ordinary room, and had to resort to a tunnel under the clay of his garden. Those who have the means and perseverance to make their own mirror, should be careful not to proceed with the finishing touches until an hour at night when their workshop, if in a town, is free from tremor.

First secure a well-ground and carefully-centred disc, let the polish be as perfect as possible before any attempt is made to figure. Care must be taken that every square or portion of the polisher is of the same consistency and temperature, that the disc may not be acted on irregularly.

To give the pitch this quality it must be well boiled and “pulled,” so that no air bubbles are in the squares, as these cause expansion or contraction as the temperature of the apartment varies, or that of the pitch and glass from friction.

It is much the best plan to keep the workshop to the same uniform temperature as the polisher was made for, allowing no draughts to pass over the mirror while working, nor the gas or lamp to be near. When the polisher is warmed—which it should be after laying aside for any time—it should be warmed equally. Neglect of the above in making the polisher, or any incautious handling of either the disc or polisher, will be sure to cause defects, which cannot be cured but by retracing the early steps in the fine grinding with an accurately centered tool.

The polishing successfully accomplished, the process of correcting for parallel rays is next proceeded with. At every step all possible care must be taken to prevent the mirror from running into an irregular curve. The importance of this cannot be too strongly urged, if a speculum capable of doing the best work is desired, as the curve must be true, regular, or uniform, to give the highest defining and illuminating power. An under-corrected mirror, if of a regular curve, will perform much better than a compound correction, exhibiting at the focal point much less lateral aberration.

A brief explanation of this may be of use as far as the limits of these pages will admit. It would be impossible to teach, by a mere description of methods, how to commence and finish off a speculum of good quality, even if every working secret and minute detail were unreservedly explained. The only way is to master sufficient theory, and the rest will come by prolonged care and perseverance. I state this because amateurs who have been desirous of enjoying the gratification of observing with a mirror of their own making have written for information which they have been quite willing even to pay for liberally, but have felt disappointed, and perhaps thought it somewhat discourteous, on being told that what they wanted was impracticable and could not be satisfactorily attempted by the Optician in writing. In some cases I have finished amateurs’ work on agreeable terms, and which has led to a pleasant correspondence or acquaintance. These remarks may prevent some future disappointment.

Every one is familiar with the fact that the parabola is the only concave surface that can reflect rays of light falling on its surface from an object at an infinite distance—such as the stars and planets practically are—to one common focus, or without aberration. This series or column of rays (which is equal in diameter to the opening of the speculum), reach the mirror without making an angle to it or to each other, they travel side by side, they are all of one length, and are reflected to a point, and are therefore all of the same length at the proper focal distance, viz., half the radius of the curvature of the concave. The properties of the parobola make the nearest possible approach to it of the utmost importance.

It has been said that a considerably under-corrected surface, if of a true and uniform curve, is better than one with less aberration, with zones or sections of various curves. To explain this, let us suppose an artificial star at some short distance, say 50 yards, the parabola would not form an image of this at the eye-piece without aberration; it has for this distance too short a focus for the central rays, and the best disc is inclined to the inner focus, because the rays from the object diverge towards it, instead of travelling parallel; and they reach the surface (the central rays compared with the marginal), at an angle equal to the semi-diameter of the mirror.

But, if instead of a parabolic mirror an elliptical one be used, which has one of its foci at 50 yards’ distance, the image will be perfect. Now place the artificial star at 500 yards, the image will now be attended with perceptible aberration. The longer focus of the ellipse must be worked further and further from the mirror by shortening the focus of the central rays. Correct it for this distance, and again remove the artificial star to a still greater distance, repeating the corrections as before and carrying the outer focus towards the object, and the inner towards the mirror, until the rays from the object become more and more parallel, and the curve is nearer and nearer to the parabola, or that eccentricity of ellipse which acts better and better for distant objects or parallel rays.

It is evident from this that if the ellipse is corrected very considerably towards the parabola, without irregularities, and every part of the surface corrected regularly from the centre to the edge (no part hastening more than another), that such a surface, though under-corrected, is much better than if one portion is fully and another under-corrected, especially if the more imperfect portion is towards the edge. Such a compound correction may show little or no outstanding aberration at the focal point, but the rays do not find a focus at the same regular pace as they would from a regular curve—one edge of a zone will be coming into focus when another would be going out. With the focussing screw they are focussed to the place where they appear to collapse, and are most satisfactory, though, in reality, the rays at the best place bend over each other, and the definition is imperfect. From a star, which is only a point, this may be more tolerated, but on the planets, where the image has a sensible diameter, and is perhaps magnified many times, this imperfect curve is very inferior to the regular one, whose error is all of one kind. The light is all there somewhere, but not with any good effect. There is no proper illumination or definition, as rays are employed which are crossing the optical axis at varying angles, and the result is confusion.

So the amateur who sets himself the pleasant task of making his own speculum (for there are many who can better afford the labour than the capital to purchase, and whose capabilities are thus superior to their means), need not be discouraged and give up the pursuit because he cannot obtain the best results by getting a perfectly parabolic glass. But if he has been successful in removing a considerable amount of the spherical error, and advanced to the elongated ellipse by maintaining a truthful curve, “let well alone” with this disc, and proceed no further, but commence another, taking care not to alter the first until the second is better, and then an improvement of the first may be attempted.

In the second attempt, should the amateur lose control over the regularity of the surface, let him try it as an experiment against the first on the planets, and he will not fail to appreciate the difference, and will be stimulated by fresh courage to get as near to the parabola as possible with the same accurate curve.

To produce a true and uniform curve is, however, the acme of troubles, whether it is desired to obtain the spherical, or parabola, or any other curve. It is generally supposed that the spherical curve is a very easy matter, so easy indeed that it is difficult to avoid. This is a very great mistake—a spherical curve of undeviating truth is as difficult a problem as a true parabola. The spherical curve is the only uniform curve, it has but one focus, and the polisher must coincide with, and be of the same radius as the glass, at every instant. This is why the optician strives to obtain a semi-polish with the grinder to lessen the risk of losing his curve on the polisher, for the curves of an object glass are spherical. The curve most liable to be obtained by the amateur is the spheroidal, a curve with its marginal rays shorter than the central, or half the radius of curvature.

There are no means with the telescope of telling the spheroidal, approximating to the sphere, from the sphere. There are no means of analyzing the exact character of a curve equal to certain methods at the centre of the curvature, but to accomplish this requires much practice and observation, with “surroundings” perfectly free from vibration. If the amateur can overcome this, and lives in or very near a town, he should only work at the polishing and figuring during the late hours of the night, when traffic, &c. have subsided. Then, by carefully preventing any draughts in the apartment, and with the mirror of the same temperature as the air in the room, he will then be able to see how varied and numerous are the chances of error in working a mirror, and the great care necessary to avoid or cure them. He will find the surface exceedingly prone to receive zones and irregularities during work, and much more so than to “work true.”

The necessity for avoiding incautious handling or heating may be realized by the following little experiment when one can manage and understand it:—Place the tip of one finger on the surface, as it hangs in the dark room ready for testing, and with very gentle pressure let it remain long enough to spell one’s name; it will then be seen that the feeble heat of the finger has, by expansion, raised a mound on the surface of the glass, and though this amount of swelling must be very small, yet it is enough to cast a shadow across the surface, as if something were laid on it, and quite ten minutes will elapse before the heat will leave this spot and the surface again become level. Now if the polisher were placed on the glass while this hillock was there, a permanent hollow would be the result. For a full account of these methods (of which Foucault was the discoverer) the reader is referred to Sir John Herschel’s and Dr. Draper’s works on the telescope.

Care must be taken not to leave too much aberration, as then the central disc is formed too positively outside the focus, and the rest of the light from the object appears as obtrusive rings and false light. The over-correction is bad, and acting as a negative lens the disc is formed too near the mirror. Such a correction, besides being objectionable on almost all classes of objects, prevents the use of the “Barlow” lens, and acts badly with all kinds of positive or Ramsden eye-pieces, which improve under-corrected, but “make bad worse” with over-corrected surfaces.

If these few and brief hints should stimulate the industrious and persevering student to make his own telescope, and thus enjoy the fruits of his own labours (which may add a relish and a pleasure to his astronomical work), they will have served the purpose for which they are written.

After a “Hint” on the choice of focus for the mirror, it only remains to say a few words about the plane, as this, with the large speculum, are the only parts that need be home-made as far as the optical work is concerned. Never adopt a “dumpy” for general use where high powers are sometimes wanted, for small and moderate sizes, say 8 to 12 diameters, and for large sizes not less than about 6 diameters of the mirror, as the larger ones practically admit of a shorter focus. The short ones can be mounted somewhat cheaper, but never choose them on this account, they will not make so satisfactory an instrument, and no adaptation of “Barlow’s” will make it so.

To make the plane, provide three well-turned metal discs of 7 inches or 9 inches diameter. Iron is the best, as the work will go down closer, and the plates or discs may be very thick—say an inch—and not so liable to “spring.” These turned discs must be scraped and ground perfectly flat on each other, until they are in good contact all over, so that there is nothing between the faces when testing them. In the earlier stage of “truing,” oil and colour can be used. When these discs are proved true, a disc of plate glass, same size, is cemented with pitch on one of them, or on a thick disc of glass, and care must be taken that it is not strained while on the block during working. Truly grind this plate on one of the tools to a fine semi-polish. Polish on the same tool with a piece of thick silk or alpaca, laid over and cemented down by a solution of resin dissolved in turpentine (as much resin as the turpentine will dissolve), then, with the tip of the finger, thinly smear over the tool and bind round the edge with cord, and the silk will keep in place. Fill up the texture of the silk or alpaca evenly with damp rouge, keep it uniformly damp, and never let the rouge and water work about. It may be polished on very hard strained pitch, but pitch for the amateur is not so safe, as it is liable in his hands to alter its form and destroy the truth of the plane, but it is the quickest and handiest if it can be managed.

After the polish is perfect remove the plate from the support and cut into squares a little larger than the size of the intended flat. Test these in the telescope on a star. If one plane turns out bad, the whole will most likely be so, and another plate must be worked with renewed energy and care, for a bad plane will spoil the action of the speculum however good, for there is no way of counteracting the curved surface of the flat or second reflector. The edge of the plate for about an inch should not be used.

When a good plane is found it can be edged by turning a piece of wood a little less in diameter than the minor axis of the plane required, turn the end square, and mark a line around it distant from the end equal to the diameter, and cut through to the opposite edge, and it will give the oval and will appear round at 45°, this will mount nicely in brass tubes of the proper size, and a cover should be made to fit easy.

“THE EQUATORIAL.”

Fig. 1 is a modification of the German principle, and it should be a sufficient recommendation to remark that it is the principle used by a maker of such experience as Mr. John Browning, and is, without doubt, the very best style of mounting for a fixed equatorial, especially when clockwork is employed, because clock power is applied to the polar axis itself direct from the driving worm.

The driving part, viz., the worm and wheel, which is out of the observer’s way (being between the standards), is not liable to meet with accident, and the driving wheel being near the lower end of the axis, is at the most rigid part, viz., the foundation of the instrument. As it has a long and very firm polar axis, and is connected with the foundation plate, it secures the utmost steadiness and freedom from strain.

An equatorial mounting, with two long and stiff shafts for the axes, has always the advantage of firmness; the holding portions of the instrument being in masses, are not liable to receive injury from blows, and thus be put radically out of order. No means should be resorted to to make an equatorial of light weight; an instrument cannot be light and slim and at the same time firm and steady—no amount of steadying rods, splines or strings, will make a slender tripod for a refractor so steady as a firmly made one, with proper size and weight in the parts.

Another important advantage of this kind of mount for a large size is, that although it has considerable weight, it is very convenient to move and set up, being built of convenient parts, which are easy of separation and removal. The uprights or standards are separate, and are bolted to the bed-plate, the upper and lower discs are readily detached from the stand. The cradle and tube are in this construction separable. The tube of the telescope being suspended over the side of the stand, is in the most convenient position for observing objects at any altitude, as the stand is out of the way and clear of the telescope. The various disadvantages and objections to the driving clock being carried by the telescope are here avoided. The clock, which is large and very powerful, is bolted down to the iron bed-plate, and the telescope, not having to carry the clock or weight, the balance is never altered nor the rate of the clock disturbed, and thus a strong clock, keeping regular time, and working much longer can be used. They will drive 2-1/2 hours. It will be readily understood that a driving clock to enable the instrument to do the most exact and best work must be powerful and a good time-keeper. Lord Lindsay has said that “the clock should have twice as much power as is used,” otherwise the spindles, &c. are pent up, and it is moreover sensible to any extra weight or work being put upon the instrument.

To give an idea of the efficiency and regularity of this clock, I may mention that a Newtonian telescope of 18 inches aperture, intended to carry a photographic camera of 112 lbs. weight, needed no additional provisions to do the extra work, the rate and power not being affected. There is a mechanism in the clock “for making up of time” and in setting the hour circle can be used with a joint handle or hooks as a fine screw-motion, and can be applied whether the clock is going or not. There is also a provision at the foundation for throwing the instrument out of the meridian, to follow the motion of the moon or planets with the clock and “maker up.” The above mounting is equally suitable for “Cassegrains” and “Newtonians,” to both of which forms it has been successfully applied up to 18 inches of aperture. It is well suited for large telescopes.

Fig. 2 is a mounting on the same principle, but with a shorter polar axis, and the column is in one casting. It is well suited for moderate sizes, and the circles, &c. are applied exactly as in Fig. 1, but when a clock is wanted there is no mounting equal to the former.

The “Educational” is a plain 6-1/2 inch, of 4 to 6 feet focus, and is made to this pattern, with revolving body and screw-motion for following. It is made as portable as it can be, and is a steady, good-working instrument, and much approved of.

THE UNIVERSAL ALT-AZIMUTH.

(Fig. 3.)

In this mounting I have seen little or no alteration needful, except that the tube is now made to balance so that the eye-tube can be reached for objects in the zenith without the observer having to elevate himself, and the elevating rod can be clamped without the lever, the legs of the stand are more curved and have more spread in them.

This “Alt-Azimuth” stand has met with much approval; and where portability is of consideration and the observer has to set up and remove the entire instrument after every night’s work, this, or the Angle-Block stand, will be found the most convenient, more so than an equatorial in any form. There is much less weight to remove, and being in three convenient parts is more readily put together and separated in the dark. If the stand and trunnions can be left in the open air a very small covering will serve to protect them, and then the tube alone has to be removed. The equatorial cannot be too highly recommended where it can be a fixture and undisturbed, as when once got into proper adjustments its advantages can then be realised, but not unless it is a permanent fixture. The circles of a portable equatorial can only be used for very rough reading, and consequently the adjustments are never in order, and the readings are of very limited use indeed.

THE IMPROVED “ANGLE-BLOCK” STAND.

(Fig. 4.)

With respect to the tube, it is mounted like the Alt-Azimuth, but with the plane of the horizontal movement corresponding to the latitude of the place of observation, and therefore following objects with one screw movement is in reality a telescope with equatorial motions. It need not be of heavier construction than the Alt-Azimuth, and there is not the double weight of counterpoising, &c. as in the equatorial, and when circles are not desired this will be the most economical, handiest, and easily managed instrument, as it partakes of the equatorial form or motions. The telescope tube is well balanced, and the declination movement is easy and free, and fitted with a clamp screw, so that when the instrument is turned on an object that object can then be followed by an endless screw.

It can be made to suit any latitude, and by the addition of foot screws on a level floor, can be got into suitable adjustments, and if it has to be removed (for the stand is very little heavier than an ordinary Alt-Azimuth), “guides” can be provided, so that it can go very approximately into the exact place again when brought out for use. They are made, when desired, with a revolving body and fine screw-motion in declination.

THE “POPULAR REFLECTOR.”

With Angle-Block Stand.—(Fig. 4.)

The speculum is 5-1/4 inch diameter, and carefully figured, and is recommended as a very useful instrument. The size and power of this telescope has been adopted as that most likely to meet the means and requirements of a large number of amateurs. Many prefer to commence astronomical observations with “something inexpensive,” and are led to begin with the popular and well advertised 3 inch refractor at £5. These, except in a few chance cases, are sure to prove disappointing, and are perhaps the cause of their giving up any further attempts to follow up the subject of astronomy, which may otherwise be so pleasantly and profitably pursued with a reliable instrument.

It is well known that a less aperture than 4-1/2 inches is insufficient to give the observer a satisfactory idea of the varied and most interesting details of the planets, and the ever-changing outline and tone of the belts and markings of Jupiter, Saturn, and Mars. Large apertures bring out details when the smaller ones can only show a general outline. The former also delineates more with a lower power, providing the focus is of proper length.

The planets and nebulæ cannot be seen to advantage without aperture and focal length. The field of view is then flat, the object is properly illuminated, with sharp and crisp definition, and is also much less subject to annoyance from tremour, through the necessity for constant adjustments in the fields, &c., as is the case with small apertures, for the object is magnified more in proportion. A certain magnifying power is necessary in order that the object may be sufficiently large to scrutinize; and this, whether the aperture is large or small, must be from about 150 to about 300 times for the planets. But 150 on a 3 inch is a high power for the quantity of light obtained, to say nothing of the separate consideration and advantage of long focus. Aperture is a quality or function of the telescope considered separately from light or focus. For instance, suppose a 5 inch speculum is so thinly silvered that it gives the exact degree of light as a 2-1/2 inch refractor, the defining and separating power of the 5 inch aperture would be very superior to the 2-1/2 inch.

The “Popular Reflector,” with 5-1/4 inch speculum of 5, 5-1/2, to 6 feet focus, will be found very suitable, and if its illuminating power is not greater than a 4-1/2 inch refractor, its defining and dividing powers are superior. By choosing the above focal lengths, according to circumstances, the observer can reach the eye-tube, while standing erect for objects in the zenith, and it is a great mistake to suppose that the shorter the tube the more handy it becomes. These foci will not require a “Barlow” lens to flatten the field. The “Barlow” is very useful for short foci when the aperture is considerable, as it improves the imperfect correction for spherical error, but this is much better corrected in the mirror itself than by a “Barlow” lens, which cannot be used without more than one disadvantage. There must, by its insertion, be some loss of light, which can ill be spared with small instruments, and when used to obtain magnifying power there is some disturbance of colour, and this subtracts from the beauty and purity of the definition of a reflector. There is nothing equal to a good eye-piece to obtain power, and flatness of field by the curve of the speculum.

With the “Popular Reflector” and an outlay of a few shillings on some popular books, such as the excellent work “Celestial Objects,” by the Rev. T. W. Webb, and some first-class publications by Mr. R. A. Proctor, especially his smaller star atlas, &c., the amateur can compare the work he is then capable of doing with a large and expensive refractor which might be beyond his reach.

THE ADJUSTMENTS OF THE EQUATORIAL.

When the inexperienced amateur purchases an equatorial with circles, he should not be without the third edition of “Chamber’s Handbook of Descriptive Astronomy,” which, besides being an excellent book in other respects, is a really practical guide to the use and application of the Equatorial, and is indispensable to the beginner. He will there find the fullest details of the adjustments to any degree of exactness. Besides many other matters he will be instructed in the use and application of apparatus to the perfect Equatorial, including all kinds of eye-pieces, micrometers, &c., &c., as well as other optical instruments and accessories.

The Equatorials described in this catalogue are provided with every means of adjustment. The cradle contains powerful screws to set the line of sight at right angles to the declination axis, and shifting screws to place the polar axis in the meridian and to the correct elevation, and with care and a few experiments with the adjustments, and by observations of some catalogue stars, the various adjustments will soon be correctly made, and the verniers set accordingly.

THE CASSEGRAIN.

This is a form of reflecting telescope but little known. This is rather strange, since it is a very much better principle than the “Gregorian,” so well known to the old observers. Herschel says it admits of a theoretically perfect telescope. Compared with the “Newtonian” it has its advantages and disadvantages. Its principal advantages are, first, the shortening of the tube, which in large telescopes is sometimes very important. Secondly, the observer has not to ascend to the eye-tube, the observations being made at the lower end, as with a refractor. The “Cassegrain” has a flatter field of view, owing to the action of the curve of the second reflector causing the rays to travel twice the distance, and, adding the element of magnifying power, the eye-pieces need not be composed of small lenses. The adjustments are perhaps a little more trouble, as the line of collimation must be carefully attended to, this requiring only a little more care can soon be accomplished, and then the definition of a good “Cassegrain” is very pleasing.

Amongst its disadvantages is the necessity for the observer to gaze upwards as with refractors, which, when the object is at a considerable altitude, is distressing, this is one of the reasons why the “Newtonian” is so pleasant to use, on account of the natural and easy position of the observer. The eye-piece being a fixture, it is not quite so convenient to use some of the accessories of the telescope. But there are means to overcome these drawbacks, and so make the “Cassegrain” even more handy than the “Newtonian.”

I have mounted an 18 inch speculum of 12 feet focus in the “Cassegrain” form, so that objects at any altitude could be observed with the greatest possible ease. A plane was fixed near the large mirror to receive the rays from the convex reflector and to throw them out to the side, illuminating apparatus were fixed here, and micrometers, &c. used, as if it were a “Newtonian”; the tube was thus made shorter, and the flat field of a long focus realised, but there would be a little loss of light in consequence of an extra reflection, but this, with a large aperture (and the fact that the “Cassegrain” gives a little more light than the “Newtonian”) can better be spared, considering the convenience gained. The observer is not elevated for any altitudes, and a large telescope is actually occupying less room than a small one. It can be used with or without the diagonal.

I have, by request, fitted the “Cassegrain” with means for two observers to view the same object at the same time. A perforated plane was arranged to receive a portion of the converging rays and throw them to the side of the tube into an eye-tube, and the remainder passing on to the eye-tube at the proper place, two images are thus formed, and can be magnified at will and viewed simultaneously. The perforated plate was so arranged that it could be removed at pleasure.

FIG. 1.

FIG. 2.

FIG. 3.

FIG. 4.

THE ALT-AZIMUTH STAND.

Fig. 3.

The Telescope, being balanced on trunnions, can be moved from an elevation approaching the zenith to an almost horizontal position. In order that it may be secured anywhere between these extremes, attached to the upper part of the telescope is an iron rod, which, sliding through the end of the arm of the stand, can be there clamped. The telescope will now be clamped in Altitude. As the progression of celestial objects will apparently be very slow, resource must be had to the smoothness of motion obtained by a screw. The upper end of the Altitude rod is therefore tapped to receive a long screw with a large milled head, jointed to the telescope body; by revolving this head the telescope is raised or depressed accordingly as the screw is unscrewed, or the reverse. It is necessary that the screw should be withdrawn some way from the rod before clamping it, preparatory to following an object which has passed the meridian, or is setting; as perhaps, just when the clearest vision is obtained, the observer may be annoyed by the screw action being suddenly stopped by the milled head coming in contact with the top of the rod.

When viewing objects near the zenith, and the focus long, the handle attached to the clamp will be found useful, as it can thereby be reached without leaving the finder. The handle may be so placed that a downward push should clamp, and an upward pull release.

The second motion in the Alt-azimuth Stand, namely, Azimuth, is obtained as follows:—The strong iron disc which forms the upper fitting of the legs has its surface accurately turned. On this revolves an iron disc, rather less in diameter, to which the trunnions which support the telescope are attached. The main axis of this disc passes through the centre of the lower disc, and then through a hollow bearing tube, a continuation of it. All these fittings having been most carefully turned and ground together, great steadiness, combined with facility of horizontal movement, is ensured. In order that this motion can be communicated as evenly as possible, resource must be again had to a screw which is thus applied. Just within the circumference of the lower disc is a narrow groove, turned to such a depth that the ring which is thus separated from the main disc is still firmly held to it by the uncut portion. An iron clamp grooved to this ring holds the nut of a long screw, the plain end of which is jointed to the upper disc. When this clamp is fixed to the ring, any motion given to the screw will act on the upper disc, and cause it to revolve, and thus the whole telescope will be slowly moved in Azimuth. The advantages of this plan are many, the most important being the rapidity and ease with which the telescope can be shifted from one object to another, even to those in contrary directions; all that is necessary being to release the clamp and turn the telescope to the object required. The clamp being carried round with the upper disc, can be fixed directly the desired position is obtained, when the screw is at once in action. Should it happen, whilst following an object, that the screw becomes exhausted from the joint and clamp coming together, the clamp should be released, and the screw turned sufficiently in the reverse way to bring it into action, the weight on the upper disc keeping the telescope meanwhile in position. If this operation be rapidly performed, the whole length of screw can be brought in play before the object has left the field of view of the finder, and can thus be easily refound with the higher power of the telescope. Motion is applied to the screw by means of a Hook’s joint, named thus from its inventor. This joint being furnished with a long handle, enables the observer, by means of it, to move the Telescope in Azimuth at any rate, and without removing his eye from the eye-piece.

It will be seen from the preceding remarks that by means of the vertical and horizontal screw motions, the telescope, when clamped, can be moved in any direction with the greatest facility, permitting a celestial object to be observed with high powers for a considerable time, and with the greatest pleasure and comfort to the observer.

Adjustments.

These instruments are always sent out in correct adjustment, and with moderate care during transit, and afterwards, will remain so, but as the performance of the instrument greatly depends on the accuracy of the adjustments the following instructions will enable the observer not only to ascertain whether they are perfect, but also to render them so if found defective. These adjustments are by no means difficult, and will be easily understood by attention to the following remarks:—

Into the draw tube screw the “adjusting piece,” which is a small brass circle with a hole in its centre about 1/20 of an inch in diameter. (The draw tube should be in about the same position as when at focus with an eyepiece.) Place the large mirror in its cell in the tube or body of the telescope, taking care that the three bayonet-joint pins are correctly placed, that is, with grooves pointing downward. They will be found to drop easily into their corresponding holes; care must however be taken that the grooves have gone well home. Both the speculum and small mirror, or “flat,” must be uncovered. On looking through the aperture of the adjusting piece, if the mirrors are in correct adjustment their reflections will be seen as follows:—the small oval mirror being placed at an angle of 45° will appear circular, and reflected exactly in the centre of this circle will be seen the bright image of the large mirror with a dark round spot in its centre, as shown by Fig. 1. This dark spot is the double reflection of the “flat,” and should be concentric with both the bright reflection of the large mirror and the circular outline of the “flat.” All these should also be perfectly concentric with the circle given by the stop in the draw tube. Should these circles not be all central the adjustments are not perfect and must be rectified as follows.

To adjust the “Flat” or small diagonal Mirror.

If the bright reflection of the large mirror is seen as a perfect circle, but not exactly in the centre of the “flat,” the latter requires adjustment; for this purpose loosen the milled head screw at the middle of the back of the “flat” which in large instruments is made sufficiently heavy to act as a counterpoise to prevent vibration. This will allow the “flat” to be rotated by the hand vertically with respect to the tube of the telescope. Bring the bright circular reflection of the mirror exactly central in this direction, and fasten in position by screwing up the milled head screw or counterpoise. If the circular reflection is quite central no further adjustment is required, but if not, then, after completing the vertical adjustment, as described, proceed to make the horizontal adjustment by turning in one or other direction the milled head of the horizontal adjustment screw, situated in front of the vertical adjustment screw; this will bring the circle of light exactly into its proper horizontal and central position, and the adjustments are then completed.

If the bright reflection of the large mirror is not seen as a perfect circle, and the small dark spot not in the centre, the speculum is out of adjustment, and consequently the adjustment of the “flat” is best performed by removing the large mirror or speculum with its cell and so arranging the body of the telescope that on looking through the “adjusting piece” a large sheet of white paper spread on the ground a short distance from the open end appears as a white circle of light reflected in the “flat.” Now bring this white circle exactly into the centre of the flat precisely in the way described above, and on this being accomplished replace the speculum uncovered with its cell, and proceed.

To adjust the large Mirror or Speculum.

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

When in perfect adjustment the large mirror viewed through the “adjusting piece” should appear as before stated, as a complete bright circle, with the image of the “flat” as a smaller dark circular spot exactly in the centre (Fig. 1). Should this not be the case it must be rectified by means of the three adjusting screws at the back of the cell. Proceed as follows:—First unloose (by a few turns) the small clamping screws which pass through the larger hollow adjusting ones; then, on looking through the adjusting piece the relative position of the image of the “flat” should be carefully noted. If the dark spot is nearer the bottom, and consequently more of the top part of the bright circle is seen (Fig. 2), the mirror reflects too much of the upper part of the tube, and therefore the top of the mirror leans too far back and must be pushed more forward by screwing in the top adjusting screw (a Fig. 1) a little at a time till the dark spot is central. Should the spot be towards the top (Fig. 2 inverted), the reverse holds good and the top adjusting screw (a) must be unscrewed or the two other adjusting screws turned in. Should the spot be towards the left (Fig. 3), screw up the right adjusting screw (c Fig. 1). If towards the right (Fig. 4), screw up the left adjusting screw (b Fig. 1). The dark spot is always furthest away from that part of the tube which is too much reflected, and from the adjusting screw that must be turned in to correct it.

When all the adjustments are considered perfect, as in Fig. 1, the small screws are to be clamped up to keep the adjusting screws in position. If, after the greatest care has been taken in adjustment, a flare should appear on looking at a star (say of the second magnitude) with an eye-piece of a high power, and the diffraction rings are not quite concentric, it can generally be rectified by turning the large screws of the mirror round in cell a little at a time. If this does not remove the flare the adjustment of the “flat”. is not sufficiently correct and must be altered by means of the screws at its back. If the flare is at the top or bottom of the star the “flat” must be very slightly revolved by the hand, after unloosing the middle screw, and when correct reclamping it. If at either side, namely, in the direction of the major axis of the “flat” and in a line with the tube, the “flat” must be altered by the long screw. It is always advisable to leave the telescope for a short time undisturbed, especially if, on first looking at a star, a flare should appear, as these appendages often vanish when the instrument has been for a short time in the air. These adjustments may at first appear somewhat difficult, but are rendered remarkably easy by observing how the different screw movements alter the positions of the reflections. A useful test as to the correctness of the adjustments may be obtained by viewing a star with a high power eye-piece out of focus. When in the centre of the field it should appear as a bright luminous circle with a circular dark spot in the centre, the size of the bright circle diminishing as the focus is approached and the dark spot remaining central.

The cell mount of the large mirror can be removed from the tube and replaced without disturbing its adjustments, but it is very advantageous if the entire instrument can always be left undisturbed when not in use in an observatory of light construction, having a skeleton revolving dome, covered with well oiled canvas or calico, and made with a wide opening and large shutters. The Rev. E. L. Berthon has described in the “English Mechanic,” October 13th, 1871, an observatory of this kind most admirably suited to shelter a reflector, as the temperature inside would be as nearly as possible that of the external air, and no annoyance from damp would be experienced. Where an observatory is not practicable the telescope might be protected by a close fitting covering of like make. Both the large and small mirrors should be protected by their covers (with which they are provided), when not in use, especially if left in the open air. The larger sized tubes have a door large enough to admit the cover and so allow of its being put on the large mirror without the necessity of the speculum being removed from the body of the telescope.

To Adjust the “Finder.”

Direct the telescope to any bright star (the Pole-star being by far the best, as it has very little apparent motion), and bring this star into the centre of the field of a low-power eye-piece. Now adjust the “finder” by means of the three screws bearing on it, till the star is bisected by the cross wires seen in the focus of the eye-lens of the “finder.” Change the low eye-piece to a high one, and perfect the adjustment as before described. Any well-defined terrestrial object at a distance can be employed in the day-time to roughly adjust the “finder,” leaving the final adjustments to be made by a star.

The use of Stops.

A stop 1/4 inch less in diameter than the speculum, is often useful, in order to cut off the internal reflections of the tube.

With mirrors 6-1/2 inches in diameter and under stops are seldom required; with the 6-1/2 inch a 5 inch stop may however sometimes be used with advantage on very bright objects. With an 8-1/2 inch mirror a 7 inch stop often improves planetary definition.

With larger sizes than 8-1/2 inches several stops of different diameter may be used, when experiment will determine what size is best suited to the condition of the atmosphere, and the character and brilliancy of the object to be observed. These stops can be easily cut out of thin cardboard and afterwards blacked with Indian-ink or lampblack.

On Observing the Sun.

No larger aperture than 4-1/2 inches should be employed for solar observation, except with a specially constructed “Solar Eye-piece,” as even a 4-1/2 inch speculum often concentrates sufficient heat to crack the coloured glass of an ordinary sun-cap, if exposed for any length of time.

If a “solar eye-piece” is employed, the whole aperture may be used. In viewing the moon a slightly tinted glass is often most useful, especially to persons with weak sight, as it takes off a great deal of the glare.

To Silver and Polish the Specula.

The cost of silvering is trifling, and with cleanliness and ordinary care very little difficulty will be experienced. The apparatus and chemicals required consist of the following articles:—

Apparatus.—A Silvering Vessel:—This should be a flat-bottomed circular glass or glazed dish, 1 inch or more larger in diameter than the speculum to be silvered, and sufficiently deep to allow of a stratum of fluid of an inch or rather more between the face of the mirror and the bottom of the dish, the top of the mirror being nearly level with the edge of the dish.

A Mixing Vessel:—A 40 oz. glass measure will answer well, but care should be taken not to scratch the sides whilst stirring, or the glass is liable to fly. Should a measure not be procurable, any receptacle of sufficient size may be used, but a glass one is best, as it will allow of the action of ammonia being better observed.

A Box of Scales and Weights.

A Glass funnel and filtering paper.

Two Glass Rods for Stirring purposes.

A Five-ounce Glass Measure.

A Test tube 3/4 or 1 inch in diameter.

Some Clean Cotton Wool.

Some very fine Wash-leather.

A Support of Wood, on which to cement the speculum, described further on.

All these articles should be perfectly clean and free from dust, and the Glass ones well rinsed with distilled water just before using.

Chemicals.—Nitrate Silver.

Potash pure, precipitated by Alcohol.

Sugar of milk powdered.

Nitric Acid, pure.

Liquor Ammoniæ.

Distilled water, pure.

Pitch.

Fine Rouge.

Turpentine.

Procure a strip of wood an inch or so less in diameter than the mirror, and of sufficient length to rest securely on the opposite sides of the silvering vessel. Pour on this piece of wood some melted pitch, and whilst it is still hot, place on it the back of the speculum moistened with a little turpentine; when cold, reverse, and lay the cemented speculum face downwards, suspended in the dish. Should the distance between the face of the mirror and the bottom of the dish be less than an inch, raise the mirror by means of thin wedges, placed between the strip of wood and the edges of the dish. But if, on the contrary, the mirror should be considerably more than an inch away from the bottom, which will occur if the silvering vessel is very deep, the speculum must be cemented to a block of wood of sufficient height, screwed to the suspending strip, instead of being cemented directly to the strip itself.

When the speculum is properly placed in the dish, namely, with the front surface about one inch from the bottom, pour in water till the fluid reaches about 1/4 inch up the side of the mirror. Measure this quantity, as it will indicate the total amount of the silvering solution required to be prepared.

To prepare the Silvering Solutions—make 3 standard solutions as follows—

Solution No. 3 must be made just before using. The others will keep if the distilled water employed in their preparation is pure, and the solutions, when made, are kept in glass stoppered bottles.

Suppose it is desired to silver an 8-1/2 inch mirror, proceed as follows:—the total amount of solution required having been ascertained, as before described, pour 2 ozs. of solution No. 1 into the mixing vessel previously well washed and rinsed with distilled water, and cautiously add Liquor Ammoniæ. A grey precipitate will be formed; continue to add the ammonia, drop by drop, till the precipitate is just dissolved and the solution becomes clear. The solution should be well stirred with a glass rod whilst adding the ammonia. Now add 4 oz. of solution No. 2, and re-dissolve the brown precipitate which is produced with Liquor Ammoniæ as before described. There will now be about 6-1/2 oz. of solution. Subtract the 6-1/2 oz. from the total amount previously ascertained to be required, and the remainder will be the amount of distilled water to be added. Add half this quantity of distilled water to the 6-1/2 oz. of solution, and add a drop at a time of solution No. 1, till there is a slight precipitate, which cannot be re-dissolved by a considerable amount of stirring (say for 2 or 3 minutes); then add the remaining half of distilled water and cover up from dust the vessel containing the solution, so as to allow the slight precipitate to settle. There is a point of importance to be attended to, namely, that no more ammonia is employed than is absolutely necessary. The total amount of ammonia required in the 6-1/2 oz. of solution is about 2 drs.

To Clean the Mirror.

Fill the end of a test tube with cotton wool, leaving plenty outside the tube. Having poured a small quantity of strong nitric acid on the front of the mirror, rub the acid well all over the front and sides with the cotton wool brush. Place the speculum under a water tap for a few minutes till the acid is washed away, and finally well rinse with distilled water; then place it in the silvering vessel (previously thoroughly cleansed) and pour in distilled water till it reaches 1/8 inch up the side of the speculum.

To Mix the Solutions.

The precipitate having settled, pour into a clean vessel all that is clear of the solution, leaving about 2 oz. behind, which will be turbid with the precipitate, and therefore useless. The total amount will be afterwards made correct by the addition of the same quantity of solution No. 3. Having filtered solution No. 3, warm it to about 100° F. by allowing the bottle to stand in warm water, or by heating in a small flask. When everything is quite ready, add 2 oz. of the filtered solution No. 3, whilst warm, to the clear solution described above, and thoroughly mix.

To immerse the Mirror.

Remove the mirror from the distilled water, taking care not to touch the surface of the mirror, and wipe the back and edge with some clean cotton wool. If this precaution is not taken the water is liable to drain down the sides of the mirror whilst silvering, and cause streaks at the edge of the film. Having poured away the distilled water in the silvering vessel, substitute the mixed solutions, and directly the solution becomes slightly inky, gently immerse the mirror, taking care that no air bubbles, or specks of any kind, remain between the surface of the mirror and the solution. The mirror should not be removed from the bath until all the silver has been exhausted from the solution. This may be known by the solution being clear below the silver film on the surface of the liquid. The time required will vary from 45 minutes on a hot summer’s day, to 90 minutes when the thermometer shows a low degree of temperature. In the latter case it is better to have the silvering bath in a warm room.

Immediately the mirror is removed from the bath, the silvered surface should be well washed by allowing ordinary water to flow on it from a tap, for five minutes or more, then finally rinse with distilled water and place the mirror to dry with the silvered surface resting on some blotting paper. It is as well if the mirror can be left undisturbed for a day or two, as the film will be firmer, but it may be polished, if desired, after drying for a few hours.

To Polish the Silvered Surface.

Make a couple of polishing pads by filling two pieces of very soft wash-leather about six inches square loosely with cotton wool, and tie them into balls. Gently remove any dust that may have settled on the film with some loose cotton wool, and then go over it with one of the pads in small circular strokes for about 15 minutes. This will consolidate the film and fit it for polishing. Spread a little of the finest rouge on a sheet of writing paper, and impregnate the other pad with it. Go over the film with the rouged rubber with the same circular strokes till it is perfectly polished, which will take another 15 minutes or so. Never commence with the rouged pad, as the surface may be injured. When once the film has been consolidated it will remain so, and can be repolished many times with the rouged pad should it get tarnished. The pads should be kept from dust in wide-mouthed bottles for future use. With care the film will last for a long time, especially if it is not allowed to get damp, and consequently the mirrors should never be brought uncovered from the cold air to a warmer temperature.

The “flat” may be silvered and polished in the same way as the speculum, using a smaller appropriate vessel for the silvering solution.

To Separate the mirror from the Wooden Support.

Should the mirror be attached directly to its support, insert a chisel between them, when one or two gentle blows will cause them to separate, but, should the mirror be cemented to a block, stand the mirror on edge, when a slight tap on the block will detach it. Scrape off any pitch that remains on the back of the mirror, using finally some turpentine to wipe it clean. Great care should be taken not to finger the film.

Martin’s process for Silvering.

Prepare four solutions of any quantity. Keep in stoppered bottles.

Solution 1.—Dissolve 175 grains of pure nitrate of silver in 10 oz. of distilled water.

Solution 2.—Dissolve 262 grains of pure nitrate of ammonia in 10 oz. of distilled water.

Solution 3.—Dissolve 1 oz. avoirdupois of pure caustic potash (prepared by alcohol) in 10 oz. of pure distilled water.

Solution 4.—Dissolve 1/2 oz. avoirdupois of pure sugar candy in 5 oz. of distilled water, then add 32 grains of tartaric acid and boil in a flask or other clean glazed vessel for 10 minutes, when cool add 1 oz. of alcohol and then dilute with distilled water, so as to make up the volume to 10 oz.

For silvering use equal parts of each, mix solutions 1 and 2 together, and 3 and 4; when the mirror is ready mix the whole together in the silvering vessel and quickly suspend the mirror.

In the summer time, if the solution cannot be kept in a very cool place, the mirror must be quite ready to be placed in the bath, as the solutions turn instantly when mixed together.

SELECTED TESTIMONIALS

11, Wellington Park Terrace, Belfast,

July 7th, 1876.

Dear Sir,—The night before last was a clear night and I got the 6-1/2 inch out, and though the air was not good when using a 3-1/4 inch refractor, I was much pleased with the performance of the mirror.

Mr. W. came up about 11 o’clock and stayed till one, we got it on Saturn, and although rather low it was really a fine sight, Mr. W. was much pleased.

The moon being near to full we could not do much with faint points of light.

Yours sincerely, W. E. Parkinson.

Mr. G. Calver.

74, Hagley Road, Edgbaston,

November 27th, 1876.

Dear Sir,—I have now got the 5-inch speculum fairly into adjustment, and I am well pleased with it. Last night, the sky being clear for a short time, I turned it on the moon, the definition of minute craters was all that could be desired. I hope to have better opportunities.

I am, Yours truly, F. G. L.

Mr. G. Calver.

74, Hagley Road, Edgbaston,

November 21st, 1876.

Dear Sir,—I have mounted the 5-inch speculum on a simple equatorial and it works well. I am sure it will turn out a fine glass and I shall not regret the time and pains I have spent over it. It is not quite in adjustment yet, when it is it will afford me a treat. I have just tried it once on the Orion Nebula and Trapezium, details of Nebula well seen, and the 5th star in Trapezium easy.

I am, Yours truly, F. G. L.

Dowlais,

December 27th, 1876.

Dear Sir,—I have thoroughly tested the speculum, and am pleased to tell you it stood its trials well.

Yours truly, D. C. C.

Mr. G. Calver.

St. Denies, Southampton,

August 14th, 1876.

Dear Sir,—With reference to your enquiry as to the performance of the 6-1/2 in. reflector. I have pleasure in stating that I am perfectly satisfied. It readily divides the test objects and shows delta Cygni with as low a power as 160. The comes to Sirius may be considered an atmospheric test, perhaps, but I have repeatedly seen it. Your stand I find very steady and convenient; altogether I can fairly say that I consider that I have an instrument of considerable power at a comparatively small cost. I now find the attempt to observe with a refractor the reverse of pleasant.

The owner of a 3-inch refractor, after using my reflector, writes that he envies me its power, ease, definition, and comfort in observing.

I am, Dear Sir, Yours truly, A. H. S.

Hilgay Rectory,

February 20th, 1877.

Dear Mr. Calver,—The sky last night cleared up in places and enabled me to try the 10-inch; there was a slight haze, but I never saw discs so perfect and neat. I felt certain it would divide anything divisible. Clouds came up and put an end to work.

I am, Dear Sir, Yours sincerely, St. V. B.

25, Hamilton Terrace, St. John’s Wood,

May 26th, 1877.

My Dear Sir,—I have much pleasure in informing you that the very first time I saw Saturn through your 6-1/2-inch reflector, the definition of the planet was far superior to anything I had observed before, even with a good 4-1/4-inch refractor; and on another occasion I was greatly pleased with the clear and easy view of the “Comes” to ε Boötis. But even without including the stars, the views of Saturn and Jupiter through my 6-1/2-inch mirror do alone (in my opinion) well repay the cost of the telescope.

Yours truly, Wm. L. Lancaster.

Mr. G. Calver.

37, Eaton Rise, Ealing,

February 13th, 1877.

Dear Sir,—I like the 18-inch speculum, and I think it as good as it can be, and if the larger one is as good I shall be delighted with it. It gives beautiful star images: I see Sirius as a brilliant dot, a glorious object without ray or flares of any kind. I never saw it so well before.

Yours truly, A. A. C.

Mr. G. Calver.

37, Eaton Rise, Ealing,

February 3rd, 1877.

Dear Sir,—I have tried the 18-inch speculum on some tests, and especially the satellites of Uranus, and it appears fine, very fine. What I have done in photography promises well.

Yours truly, A. A. C.

Mr. G. Calver.

Southend on Sea,

March 17th, 1877.

My Dear Sir,—Circumstances have prevented me from making many observations of late, but I am more confirmed in my opinion that my telescope of your make is a very fine instrument. (A 6-1/2-inch.) I heartily wish you success, which I feel sure you will obtain, as you are so painstaking and turn out a thoroughly reliable article. I shall be curious to compare my brother’s telescope with mine.

Sincerely yours, J. L. L.

Mr. G. Calver.

Gorlestone,

March 30th, 1875.

Dear Sir,—You have asked me for my opinion of the 10-inch speculum. I have tested it, and can speak in the highest terms of its performance. Definition in good air is as near perfection as it is possible to imagine. Such tests as ζ Cancri, γ² Andromedæ are well divided. The 6th star in the trapezium stands out well clear of its brighter neighbour. On the moon any power may be applied, only lessening the light, but retaining sharpness of outline. I have tried a good many telescopes, but never yet saw these 10-inch mirrors surpassed. The image of a star disc expanded on each side of the focus is of as nearly the same appearance as possible.

Yours truly, W. P. Matthews.

416, Brixton Road,

April 20th, 1875.

Dear Sir,—In compliance with your request, I send particulars of my observation on φ Draconis. It was about 12.45 this morning when I directed my 8-1/2-inch to this object, the air was very unsteady, in fact not nearly so good as some nights lately, and I well divided it with a power of 450. I then used a 6-1/2-inch stop, which I found very much increased the steadiness of definition.

Yours very respectfully, P. H.

10th July, 1874.