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—
| No. 1.{ | Nitrate Silver in crystals | 100 | grains. |
| Distilled water | 4 | oz. | |
| No. 2.{ | Potash pure by alcohol | 1 | oz. |
| Distilled water | 25 | oz. | |
| No. 3.{ | Powdered sugar of milk | 1/2 | oz. |
| Distilled water | 4 | oz. |
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.
Dear Sir,—I tried the 6-1/4-inch mirror I had from you for the first time last night. The night was not a good one for definition, but I was very much pleased indeed with its performance.
Yours truly, C. H. W.
Honiton Rectory,
Dec. 23rd, 1874.
Dear Sir,—I have been wishing to tell you how pleased I am at the performance of your mirror. Many thanks for your kind offer to change the mirror if needful, but I think it could hardly be better than it is, its performance on different double stars is most excellent. I have examined nearly 100 of these since September with the 6-1/2-inch, most of them very difficult, but the mirror came out well under all tests. I send you a list of some of the objects I have examined.
Yours truly, H. Sadler.
π Aquilæ.—Divided, power 80. 14-m. Webb might have been rated 15-m. [6-m., 7-m. 1″·5.]
Regent Road, Great Yarmouth,
October 24th, 1874.
Dear Sir,—Doubtless you have anticipated hearing from me, relative to the 10-inch telescope, but the weather has been so indifferent for delicate astronomical work that it has not yet had a fair trial.
In middling good air, when I could use a reduced aperture with satisfactory results, the star images reminded me of the 6-1/2-inch, which I considered so superb a mirror that nothing could surpass its fine performance either on stars or planets.
With the recollections of the truly splendid views I had of Jupiter last season, I count much to see him with the 10-in.
Yours truly, H. Blyth.
Regent Road, Great Yarmouth.
February 2nd, 1875.
Dear Sir,—In reply to yours, I have not had a really good night to test severely the defining powers of the 10-inch mirror, but judging from the few difficult objects I have seen, the mirror promises well, and I have no doubt of its excellence.
I had a fine view of the grand nebula in Orionis, and noticed that the 5th and 6th stars were quite plainly seen, when the aperture was reduced to 5-1/2-inches. I consider your stand a great improvement, being very convenient and steady.
Yours truly, T. Ayers.
CATALOGUE
OF
Silvered Glass Reflecting Telescopes, &c.,
AND THEIR ACCESSORIES,
JUNE, 1877.
G. Calver, while introducing the list of prices below, has the greatest confidence in calling the attention of the practical astronomer and the amateur to the very moderate prices charged, considering the principle and style of the mounting, and the firm and well-fitted arrangements in all the parts.
The principle of the mounting is that which is the most convenient to use, and with the greatest degree of steadiness with the easiest movements. Fig. 1 is the best mounting ever applied to the Reflecting Telescope.
He is also able to state, that with his processes and special facilities for working large specula, and substantially mounting them, he is prepared to construct automatic equatorials of large sizes, and with any special arrangements that may be required.
Silvered Glass Equatorial Telescopes, very substantially and well-fitted, as (Fig. 2). All these equatorials have revolving body.
| £ | s. | d. | |
| 5-1/4 inch Speculum from 4 to 6 feet focus, with 8 inch hour circles reading to 5 seconds of time, and declination circle reading to 1 minute, two powers | 30 | 0 | 0 |
| 6-1/2 inch Speculum of from 5 to 6-1/2 feet focus, 10 inch rotating hour circle, reading to 5 seconds of time, and declination circle to 1 minute of arc, with 3 powers—100 to 500 | 45 | 0 | 0 |
| 8-1/2 inch Speculum as above with 12 inch circles, 4 powers | 70 | 0 | 0 |
| 9-1/4 inch Speculum as above with 12 inch circles, 5 powers | 78 | 0 | 0 |
| 10 inch Speculum as above with 12 inch circles, 5 powers | 90 | 0 | 0 |
| 12-1/2 inch Speculum as above with 12 inch circles, 5 powers | 120 | 0 | 0 |
Any of the above sizes mounted on Stand (Fig. 1), from 5 per cent. extra.
Fig. 1 is a very excellent stand, and admirably suited for large instruments with clock power, and is convenient to remove and adapt to different latitudes, &c., see [p. 20.]
| £ | s. | d. | ||
14, 15, to 16 inch Speculum mounted as (Fig. 1), with rotating hour circle reading to 5 seconds and declination to 1 minute (the diameter of the circles never less than that of the Speculum and often larger) | ||||
8 powers from 50 to 700, with first-rate and very powerful drivingclock, first-class instrument, and complete | from £220 to | 330 | 0 | 0 |
17, 18, to 20 inch Speculum, with 10 powers, from 50 to 800, includingKellners, Huyghenians, and Achromatics, | from £350 to | 500 | 0 | 0 |
22 to 24 inch Speculum | from £550 to | 750 | 0 | 0 |
30 inch Speculum, with 12 powers and position micrometer, andtransit eye-pieces | 1000 | 0 | 0 | |
Prices will be forwarded for special arrangements in any of these large sizes.
The Cassegrain Telescope is a very convenient form, and perfect in performance, and in this form a very large instrument requires comparatively a much smaller observatory, and is more conveniently worked. They can be made so that the image is thrown out at the side and viewed diagonally, as with the Newtonian, see [p. 35.] Cassegrains from 5 per cent. extra.
| £ | s. | d. | ||||||
| Driving | Clocks for | 6 | to | 10 | inch | 35 | 0 | 0 |
| 〃 | 〃 | 12 | 〃 | 18 | 〃 | 40 | 0 | 0 |
| 〃 | 〃 | 20 | 〃 | 30 | 〃 | 50 | 0 | 0 |
The Educational Reflector is a plain and very steady and satisfactory instrument, mounted on (Fig. 2) stand, without circles, has revolving body, and made so as to be portable.
| £ | s. | d. | |
| 6-1/2 inch Speculum (clear aperture), and of 4 to 6-1/2 feet focus, has screw-motion in right ascension, 2 powers | 25 | 0 | 0 |
| 8-1/2 inch speculum as above, with 3 powers | 40 | 0 | 0 |
Any of these equatorials can be, according to wish, fitted without the circles, &c., and dispensing with that which is not needed, so as to reduce the price to suit circumstances and convenience.
The Popular Reflector, with Angle-Block Stand, with endless screw-motion to follow the stars with equatorial motion.
| £ | s. | d. | |
| 5-1/4 inch Speculum, of from 4 to 6 feet focus, with 2 powers | 15 | 15 | 0 |
(See [p. 20, Fig. 4]).
Reflecting Telescope on Alt-azimuth Stand, as described at [page 13], fitted with Silvered Glass Speculum, and provided with Two Eye-pieces.
| £ | s. | d. | |||||||
| 5-1/4 | inch | Speculum, | 4 | to | 6 | feet focus | 19 | 10 | 0 |
| 6-1/2 | 〃 | 〃 | 4 | 〃 | 6-1/2 | 〃 | 24 | 0 | 0 |
| 8-1/2 | 〃 | 〃 | 〃 | 6-1/2 | 〃 | 33 | 10 | 0 | |
| 10 | 〃 | 〃 | 〃 | 7 | 〃 | 45 | 0 | 0 | |
| 12-1/2 | 〃 | 〃 | 〃 | 8 | 〃 | 70 | 10 | 0 | |
Reflecting Telescope on Cradle Stand, and provided with Two Eye-pieces.
| £ | s. | d. | ||||||||
| 5-1/4 | inch | Speculum, | 4 | to | 6 feet focus | 21 | 0 | 0 | ||
| 6-1/2 | 〃 | 〃 | 4 | 〃 | 6-1/2 or 5 | feet | focus | 27 | 0 | 0 |
| 8-1/2 | 〃 | 〃 | 〃 | 6-1/2 | 〃 | 〃 | 36 | 0 | 0 | |
| 10 | 〃 | 〃 | 〃 | 7 | 〃 | 〃 | 49 | 0 | 0 | |
| 12-1/2 | 〃 | 〃 | 〃 | 8 | 〃 | 〃 | 76 | 0 | 0 | |
SILVERED-GLASS SPECULA (unmounted).
THE FINEST QUALITY GUARANTEED.
| £ | s. | d. | ||||||
| 5-1/4 | inch diameter | 4 | to | 6 | feet focus | 5 | 0 | 0 |
| 6-1/2 | 〃 | 4 | 〃 | 6-1/2 | 〃 | 6 | 5 | 0 |
| 8-1/2 | 〃 | 〃 | 6-1/2 | 〃 | 13 | 0 | 0 | |
| 10 | 〃 | 〃 | 7 | 〃 | 25 | 0 | 0 | |
| 12-1/2 | 〃 | 〃 | 8 | 〃 | 38 | 10 | 0 | |
| 14 | 〃 | 〃 | 8 | 〃 | 55 | 0 | 0 | |
| 16 | 〃 | 〃 | 10 | 〃 | 75 | 0 | 0 | |
| 18 | 〃 | 〃 | 12 | 〃 | 100 | 0 | 0 | |
| 20 | 〃 | 〃 | 12 | 〃 | 130 | 0 | 0 | |
| 22 | 〃 | 〃 | 15 | 〃 | 165 | 0 | 0 | |
| 24 | 〃 | 〃 | 18 | 〃 | 200 | 0 | 0 | |
| 26 | 〃 | 〃 | 〃 | 250 | 0 | 0 | ||
| 30 | 〃 | 〃 | 〃 | 300 | 0 | 0 | ||
| 36 | 〃 | 〃 | 〃 | 400 | 0 | 0 | ||
SILVERED-GLASS DIAGONAL MIRRORS (unmounted).
FINEST QUALITY GUARANTEED.
| £ | s. | d. | ||
| 1 inch | in the minor axis, or narrowest diameter of the ellipse | 1 | 0 | 0 |
| 1-1/2 | dittodittoditto | 1 | 10 | 0 |
| 2 | dittodittoditto | 2 | 0 | 0 |
| 2-1/2 | dittodittoditto | 2 | 10 | 0 |
| 3 | dittodittoditto | 3 | 3 | 0 |
| 3-1/2 | dittodittoditto | 4 | 4 | 0 |
| 4 | dittodittoditto | 5 | 5 | 0 |
SILVERING AND POLISHING SPECULA.
| £ | s. | d. | |||
| 5 | inch | 0 | 6 | 0 | |
| 6-1/2 | 〃 | 0 | 8 | 0 | |
| 8-1/2 | 〃 | 0 | 10 | 6 | |
| 10 | 〃 | 0 | 15 | 0 | |
| 12-1/2 | 〃 | 1 | 1 | 0 | |
| Diagonal Planes 3s. to | 0 | 5 | 0 | ||
ASTRONOMICAL EYE-PIECES.
OF BEST QUALITY.
Huyghenian Construction of the following magnifying powers on a 6-1/2 feet focus object-glass:—
Good Instruments will be taken in exchange and liberally allowed for.
GEORGE CALVER,
HILL HOUSE, WIDFORD,
CHELMSFORD, ESSEX.
APPENDIX
TO
Hints on Silvered Glass Reflecting Telescopes.
The object of this pamphlet is to give some new and additional advice not contained in my “Hints on Silvered Glass Reflecting Telescopes,” on silvering and adjusting them; and, as the results of continued and recent experience, it is hoped they may be useful, and may prove an acceptable appendix to my little book of Hints.
The first thing to do after fastening the mirror to the wooden support, is to suspend it in the dish in which it is to be silvered, and so to adjust it that there shall be one inch between the bottom of the mirror and the bottom of the dish; then pour in water to come a quarter of an inch up the sides of the glass—the quantity thus found is to be measured, and will be the exact amount of the bath when all the solutions are mixed; this will prevent any hitch at an important moment, and the glass can be immersed without delay or disturbance.
Next proceed to wash the surface with nitric acid, taking care that the acid does not run down the sides, as it is not so easily removed from the fine ground sides as it is from the polished surface. After gently but thoroughly rubbing the surface, add a little water, and again go over; then wash all off, and take a large piece of cotton wool and well sponge the surface and sides with plenty of water, and suspend in a dish or plate with water in it.
TO PREPARE THE SOLUTIONS.
The plan I adopt is this,—I dissolve a large quantity of the chemicals required in one-fourth or one-eighth the quantity of water employed by Martin, so that in a Winchester quart stoppered bottle I can keep a large supply ready. I make the silver and ammonia solutions eight times the original strength, the potash and sugar solutions four times only, as these latter solutions require more water to properly prepare them.
Enough solutions to silver a 6-1/2 in. mirror, eight times are to be made.
Dissolve 1400 grains of nitrate of silver in 10 ounces of water (it may be clean fresh rain water filtered, if distilled is difficult to obtain, and it will act very well), but do not put the silver into the exact 10 ounces, but make up to exact 10 ounces after the silver is dissolved. It is thus condensed eight times. Do the same with the nitrate of ammonia, by dissolving 2096 grains.
Next prepare the potash and sugar solutions, condensed to four times the initial strength.
Dissolve 8 ounces of potash, and make up to exactly 20 ounces of water; if this is dissolved in a glass measure it will evolve sufficient heat to break it; it is safely done in a clean white jug. Lastly, dissolve 4 ounces of white sugar candy with 416 grains of tartaric acid, and boil ten minutes in a clean glazed vessel; when cold, add 8 ounces of alcohol, and make up to 20 ounces with water.
Do not use the solutions till all are of the same temperature, nor on the same day as they are made; and do not silver until the glass, and everything used are of the same temperature. To insure this, get the mirror ready and suspend in water, with the solutions all collected in the room in which the silvering is to be done, and let them remain until next day.
Now, remembering how many ounces were needed to leave one inch of solution under the surface and a quarter of an inch up the sides of the 6-1/2 in. mirror, using a vessel about 2 inches larger in diameter than the mirror, pour into a glass measure 10 drams of the silver solution, add next 10 drams of the ammonia, then 20 drams of potash; if the potash turns the mixture thick, pour it backwards and forwards into another vessel, or stir it with a strip of glass, or a glass rod, for half a minute; if the mixture does not turn clear (which it will not do if it has turned very thick on adding the potash), add cautiously, drop by drop, some of the ammonia solution, agitating it till it just clears; do not filter unless there are a great many floating particles through using unfiltered water, the floating particles cause minute black spots, which are, however, of little consequence, as their action is only the loss of so much light. Measure 20 drams of the sugar—and the mirror being ready to dip (the amount with the sugar added must be the required quantity previously ascertained), add the sugar, stir well, and immerse when the mixture begins to turn dark ink colour.
When silvered, well wash the mirror with water of the same temperature as itself (it may be safely sponged with a lump of cotton wool), and stand it on its edge on blotting-paper to dry. Drops of water standing long on it when drying will cause stains, which will not readily polish off, and indeed these should not be polished, as it is not right to polish one part more than another; so it is best to leave them, or if this is objected to, re-silver the mirror, when standing it in the sun and wind will soon dry the surface, but be careful not to let the dust settle on it.
Do not warm anything; if the weather is cold, everything may be left many hours in a warm room, but the equal temperature is of the utmost importance—the actual temperature is not of much consequence. More failures are due to unequal temperature than any other cause in the hands of the amateur. One careful and clever amateur informed me that he had failed six times, but on the seventh, by attending to the advice given above, the silvering was a perfect success, and all seemed then as easy and certain as it had before seemed uncertain and difficult.
THE FINAL ADJUSTMENT OF A NEWTONIAN REFLECTOR ON A STAR.
The adjustments being as near correctness as can be seen by the usual method of procedure—of which the fullest details are given in my little book, the telescope is ready to try on a star. If all does not appear satisfactory on first observing the star, do not disturb the adjustments for a while, but wait until the air is steadier, when perhaps it will be quite satisfactory. If not, turn it on a bright star; if the adjustments are at fault, the black shadow of the flat, when the star is out of focus, will not be central in the slightly expanded image, but will be on one side of the expanded disk. Use a power of 200 to 300.
If the black spot is near one of the sides corresponding with major axis of flat, the side screw will put it right. We will assume that the black spot is nearer the edge of flat nearest the mirror, and that the eyepiece is beyond the focus proper for the star; then unscrew the side screw. If the spot is nearer the other edge of flat, screw it up little by little, taking the hand out of the telescope each time, and see what the effect is. The movement and alteration can be watched while the hand is in the tube. If the spot is seen either the top or bottom of the flat that is in the direction of the diameter of tube, the middle screw must be slightly loosened and the flat revolved. Let the screw just bite so as to hold, and tighten it when correct. The definition of a star should now be perfect, and by daylight observe the adjustments; it may appear that the reflected circles are not concentric, but if the definition on a star is good in still air, consider the appearances correct adjustments, except that the reflection of the black spot on mirror may be made concentric. It sometimes happens that a persistent and rigid centreing of the reflections on face of flat are not the positions for best definition, perhaps from a slight optical eccentricity somewhere in the instrument.
The adjustments of a reflector are soon familiar and easy, and there is this to be said, the adjustments can be manipulated upon to any amount without the slightest injury to the instrument; there is no danger of any sort, and the instrument can be perfected in its adjustments by daylight, and this makes it both pleasant and convenient to leisurely work at, for experience sake. The instrument need not be out of doors either, unless an artificial star from a black pin’s head—which is better in strong sunlight than a thermometer bulb—is to be used instead of a star at night, if so let it be as high as possible.
Be careful not to screw up the small screws too tight, as the fine threads are liable to be spoiled and the screw made useless.
THE DURABILITY OF SILVER FILMS.
It sometimes happens that the films do not last so long as they are expected to do, that is, they sometimes lose their splendid lustre sooner than they should do. When it is remembered that though the process of silvering is an easy operation to perform, yet it is a delicate chemical one, in which good results are best obtained when several good conditions all meet together, this is not to be wondered at, but it is certain the instructions given above will reduce the chances of failure to a minimum, and imperfect results will be rare. The silver usually lasts a long time. I know silver films that have been in use for ten years.
Do not suppose that a long and badly worn surface will fail to show the detail and colours of the planets, the fullest detail also of the moon or the stars. It is surprising how long a silvered surface will continue to do its full work while its appearance is much deteriorated; picking up faint points of light that are near the very limit of its aperture and power, seem alone to require the perfect and fresh film. I know silver films that have been in use for seven years and have not been ever re-polished.
That the Silvered Glass Reflecting Telescope is giving satisfaction and is capable of performing the best work of a telescope—being durable, convenient, and perfectly efficient—may be gathered from the selection of a very large number of satisfactory and most gratifying letters received by the writer; and it has been most encouraging to find in all cases the observer is most eager to express his entire satisfaction, and especially so where so many have been prejudiced, in the absence of experience, with feelings of doubt as to their complete efficiency, and many, again, having used fine refractors.
It will be seen, too, that many have commenced with a small sized reflector, and gone on increasing their optical power until they have obtained a large and powerful instrument, imposing indeed as compared with what was considered a powerful instrument less than half a century ago, and at a fraction of its cost.
The gratifying success which I have achieved, attested by the universal satisfaction given by the instruments supplied by me, is the result of unwearying labour and untiring patience; every speculum, large or small, being figured with my own hands.
I have every facility for setting up instruments, and for testing, working, and regulating them in every part on celestial objects. The optical parts are, therefore, not merely tested separately and subsequently mechanically adjusted, but each and every instrument is put together and optically and mechanically tested, as a whole, before it is permitted to leave the workshop.
Every speculum is most carefully figured, and confidently guaranteed perfect to the extreme edge.
The employment of “stops” of any kind is quite unnecessary, except on bad nights, when the aperture must of course be suited to the degree of steadiness of the air. For, whether the instrument be a reflector or a refractor, the aperture and power used must of necessity be limited by the atmospheric conditions under which observations are made.
It should, however, be remembered that the reflector having not only a much larger aperture than a refractor of same focal length, and, being open to the influence of the external air, is not only affected by a relatively larger column of air, but is affected in a different way. It frequently happens that reflectors of 6 in. to 18 in. aperture, are of same focal length as 5 or 6 inch refractors.
These considerations will explain the occasional use of stops, and the expediency of having them ready to use when they can be of temporary advantage. There are nights in our climate when a 6 inch aperture may be the largest that can be used with satisfaction or advantage; indeed it used to be accepted as an incontrovertible fact that an aperture of G inches was the largest that could be used on average nights in our country. But there are also not a few nights when large apertures, bring great gain to their fortunate possessors.
It is obvious that, mutatis mutandis, these conditions must affect refractors as well as reflectors, with perhaps this difference, that in the case of a reflector the rays pass down the tube and infringe on the speculum as a column of parallel rays; consequently all the external rays of the column travel near the surface of the tube for its entire length. This being so, it is obvious that until the internal and external air and the metal tube have time to equalize in temperature, it will be advantageous, in all work requiring high powers, such as the examination of difficult double stars, to “stop off” for a time the extreme edge by a small diaphragm.
This will occur especially on frosty nights after a warm fine day. When the telescope is in the open air, ice will not infrequently form on the top of the tube, and the effect will be at once obvious in the irregularity of a star image at the edge corresponding to the top of the tube. The use of a small diaphragm will at once obviate this defect and give perfect images. On such nights a wooden tube is preferable to a metal one. It has been frequently my experience, when finally testing an instrument, that when I have been dissatisfied with its performance, and in order to trace the cause of the apparent defect, have removed the mirrors to my wooden testing tube, I have found every defect removed, and imperfect replaced by perfect images. The explanation is simple. Not only is the wooden tube much larger in diameter than the specula, but wood is of itself less sensible than metal to differences of temperature.
To make the tubes some 2 or 3 inches larger in diameter than the speculum would cure this evil, but it would involve various inconveniences and expenses in structure, which would more than outweigh the advantage of overcoming a hindrance which is after all only temporary and occasional.
The experiment of perforating or ventilating tubes has now been thoroughly and exhaustively tried, with the result that there is a fairly general consensus of experienced opinion against their use. The advantages are merely theoretical; the disadvantages are grave. Among them are the admission of cross reflections in every direction, the admission of dust, imperfect protection of the mirrors, and a decided loss of strength and rigidity to the tube.
CATALOGUE
OF
Silvered Glass Reflecting Telescopes, &c.,
AND THEIR ACCESSORIES,
OCTOBER, 1880.
G. Calver, while introducing the list of prices below, has the greatest confidence in calling the attention of the practical astronomer and the amateur to the very moderate prices charged, considering the principle and style of the mounting, and the firm and well-fitted arrangements in all the parts.
The principle of the mounting is that which is the most convenient to use, and with the greatest degree of steadiness with the easiest movements. Fig. 1 is the best mounting ever applied to the Reflecting Telescope.
He is also able to state that, with his processes and special facilities for working large specula, and substantially mounting them, he is prepared to construct automatic equatorials of large sizes, and with any special arrangements that may be required.
Fig. 1 is a very excellent mounting, and admirably suited for large instruments, especially when clock-power is applied; and as now manufactured by G. Calver, is the most complete and reliable, its details of construction being such as to give the greatest freedom of motion and steadiness. For large sizes the top of polar axis works on friction rollers; and, as a superior German stand, its arrangements and means of astronomical as well as optical adjustments, are such as to make it an instrument of precision.
To ensure these qualities many well considered arrangements—entailing careful and expensive workmanship—must be provided; details that do not easily admit of description in an ordinary catalogue, or can be shown in an engraving, but are duly appreciated by the observer, who will find their value by practice.
The cradle bar is compound with fine screw movement for perfecting collimation, but to make this adjustment perfect, the telescope tube must be centred in a powerful lathe, and the cradle and solid metal rings in which the tube revolves ([see illustration]) has to be turned and fitted with true flanges or working bearings; the mechanical and optical centres will then coincide.
The hour circle, to be truly divided, must be truly made, and is a solid wheel of good substance, working on a secondary axis, and rotated by mechanical means, truly concentric, with a very strong polar axis.
The declination circle has fine tangent screw movement, with double action for setting the readings by milled-headed screw, and also long driving rod from the eye-piece to set and adjust the object in centre of field. There is slow hand motion, by a separate wheel in right ascension.
The clock-power is connected and disconnected instantly, by touching with the finger a small lever, conveniently placed at a small door in the clock case. The clock will go at exactly the same rate, whether it is driving the telescope or not, and the latter begins to move at the proper speed the instant the lever is moved, and the connection of the telescope and clock made, which is instantly effected by another lever.
| £ | s. | d. | ||
14, 15, to 16 inch Speculum mounted as (Fig. 1), with rotatinghour circle reading to 5 seconds and decimation to 1 minute(the diameter of the circles never less than that of theSpeculum, and often larger) | ||||
8 powers, from 50 to 700, with first-rate and very powerful drivingclock, first-class instrument, and complete | from £250 to | 350 | 0 | 0 |
17, 18, to 20 inch Speculum, with 10 powers, from 50 to 800, includingKellners, Huyghenians, and Achromatics, | from £360 to | 500 | 0 | 0 |
22 to 24 inch Speculum | from £600 to | 700 | 0 | 0 |
30 inch Speculum, with 12 powers and position micrometer, andtransit eye-pieces | 1000 | 0 | 0 | |
Prices will be forwarded for special arrangements in any of these large sizes.
| £ | s. | d. | ||||||
| Driving | Clocks for | 6 | to | 8 | inch | 35 | 0 | 0 |
| 〃 | 〃 | 12 | 〃 | 18 | 〃 | 40 | 0 | 0 |
| 〃 | 〃 | 20 | 〃 | 30 | 〃 | 50 | 0 | 0 |
These are fitted with governor, regulator, and self-adjusting break. They are made of gun metal and steel, and every wheel is cut. They drive with most excellent regularity, and are, when desired, made to beat seconds on a bell. The whole is enclosed in a mahogany frame, with glass panels.
Fig. 2 admits of equal completeness, &c., but the stand is not so convenient for clock-power.
Silvered Glass Equatorial Telescopes, very substantially and well fitted as (Fig. 2). All these equatorials have revolving body.
| £ | s. | d. | |
| 5-1/4 inch Speculum, from 4 to 6 feet focus, with 7 inch hour circles, reading to 5 seconds of time, and declination circle reading to 1 minute, 2 powers | 30 | 0 | 0 |
| 6-1/2 inch Speculum, of from 5 to 6-1/2 feet focus, 10 inch rotating hour circle, reading to 5 seconds of time, and declination circle to 1 minute of arc, with 3 powers—100 to 500 | 45 | 0 | 0 |
| 8-1/2 inch Speculum, as above, with 10 inch circles, 4 powers | 70 | 0 | 0 |
| 10 inch Speculum, as above, with 10 inch circles, 5 powers | 90 | 0 | 0 |
| 12-1/2 inch Speculum, as above, with 10 inch circles, 5 powers | 120 | 0 | 0 |
The Educational Reflector is a plain and very steady and satisfactory instrument, mounted on (Fig. 2) stand, without circles, has revolving body, and made so as to be portable.
| £ | s. | d. | |
| 8-1/2 inch Speculum as above, with 3 powers | 40 | 0 | 0 |
The Popular Reflector (Fig 3), with Angle-Block stand, with endless screw-motion to follow the stars with equatorial motion.
| £ | s. | d. | |
| 5-1/4 inch Speculum, of from 4 to 6 feet focus, with 2 powers | 15 | 15 | 0 |
These are also made with rotating body.
Reflecting Telescope on Alt-azimuth Stand, fitted with silvered glass Speculum, and provided with two eye-pieces.
| £ | s. | d. | |||||||
| 5-1/4 | inch | Speculum, | 4 | to | 6 | feet focus | 19 | 10 | 0 |
| 6-1/2 | 〃 | 〃 | 5 | 〃 | 6-1/2 | 〃 | 25 | 0 | 0 |
| 8-1/2 | 〃 | 〃 | 〃 | 6-1/2 | 〃 | 33 | 10 | 0 | |
| 10 | 〃 | 〃 | 〃 | 7 | 〃 | 50 | 0 | 0 | |
| 12-1/2 | 〃 | 〃 | 〃 | 8 | 〃 | 70 | 10 | 0 | |
Silvered-Glass Specula (unmounted).
THE FINEST QUALITY GUARANTEED.
| £ | s. | d. | ||||||
| 5-1/4 | inch diameter, | 4 | to | 6 | feet focus | 5 | 0 | 0 |
| 6-1/2 | 〃 | 4 | 〃 | 6-1/2 | 〃 | 6 | 5 | 0 |
| 8-1/2 | 〃 | 〃 | 6-1/2 | 〃 | 13 | 0 | 0 | |
| 10 | 〃 | 〃 | 7 | 〃 | 25 | 0 | 0 | |
| 12-1/2 | 〃 | 〃 | 8 | 〃 | 38 | 10 | 0 | |
| 14 | 〃 | 〃 | 8 | 〃 | 55 | 0 | 0 | |
| 16 | 〃 | 〃 | 10 | 〃 | 75 | 0 | 0 | |
| 18 | 〃 | 〃 | 12 | 〃 | 100 | 0 | 0 | |
| 20 | 〃 | 〃 | 12 | 〃 | 130 | 0 | 0 | |
| 22 | 〃 | 〃 | 15 | 〃 | 165 | 0 | 0 | |
| 24 | 〃 | 〃 | 18 | 〃 | 200 | 0 | 0 | |
| 26 | 〃 | 〃 | 〃 | 250 | 0 | 0 | ||
| 30 | 〃 | 〃 | 〃 | 300 | 0 | 0 | ||
| 36 | 〃 | 〃 | 〃 | 400 | 0 | 0 | ||
Silvered-Glass Diagonal Mirrors (unmounted).
FINEST QUALITY GUARANTEED.
| £ | s. | d. | ||
| 1 inch | in the minor axis, or narrowest diameter of the ellipse | 1 | 0 | 0 |
| 1-1/2 | dittodittoditto | 1 | 10 | 0 |
| 2 | dittodittoditto | 2 | 0 | 0 |
| 2-1/2 | dittodittoditto | 2 | 10 | 0 |
| 3 | dittodittoditto | 3 | 3 | 0 |
| 3-1/2 | dittodittoditto | 4 | 4 | 0 |
| 4 | dittodittoditto | 5 | 5 | 0 |
Silvering and Polishing Specula.
| £ | s. | d. | |||
| 5 | inch | 0 | 6 | 0 | |
| 6-1/2 | 〃 | 0 | 8 | 0 | |
| 8-1/2 | 〃 | 0 | 10 | 6 | |
| 10 | 〃 | 0 | 15 | 0 | |
| 12-1/2 | 〃 | 1 | 1 | 0 | |
| Diagonal Planes 3s. to | 0 | 5 | 0 | ||
ASTRONOMICAL EYE-PIECES.
OF BEST QUALITY.
Huyghenian Construction of the following magnifying powers on a 6-1/2 feet focus object-glass:—
| £ | s. | d. | ||
| 35, 60, 90, 130, 180, 200 | each | 0 | 15 | 0 |
| 320, 450 | 1 | 1 | 0 | |
| 610 | 1 | 7 | 6 | |
| All fitted with Sunshades. | ||||
| Improved Achromatic, Ramsden’s Construction. | ||||
| £ | s. | d. | ||
| 90, 130 | each | 1 | 0 | 0 |
| 189, 250 | 1 | 10 | 0 | |
| 320, 450 | 1 | 12 | 6 | |
| 610, 750 | 2 | 2 | 0 | |
| 850 | 2 | 2 | 0 | |
| Kellner’s Construction. | ||||
| £ | s. | d. | ||
| 30, 58, and 80 | each | 1 | 5 | 0 |
| Solar Eye-Pieces. | ||||
| £ | s. | d. | ||
| Solar Diagonal, with true plane | 1 | 12 | 6 | |
| Barlow’s Lens | 1 | 1 | 0 | |
| Day Erecting Eye-piece | 1 | 5 | 0 | |
| Coloured Eye-piece cap for observing the Moon | 0 | 4 | 0 | |
| Transit Eye-pieces, for use with Equatorials | £1 1s. to | 2 | 2 | 0 |
| Adjusting Piece | 0 | 2 | 6 | |
Good Instruments will be taken in exchange, and liberally allowed for.
GEORGE CALVER,
HILL HOUSE, WIDFORD,
CHELMSFORD, ESSEX.