PHOTOGRAPHIC CORRESPONDENCE.
Test for a good Lens.—The generality of purchasers of photographic lenses can content themselves with merely the following rules when they buy. It ought to be achromatic, i. e. consisting of the usual two pieces of crown and flint glass, that its curves are the most recommended, and that it is free from bubbles: to ascertain the latter, hold the lens between the finger and thumb of the right hand, much as an egg-merchant examines an egg before a strong gas flame, and a little to the right of it; this reveals every bubble, however small, and another kind of texture like minute gossamer threads. If these are too abundant, it should not be chosen; although the best lenses are never altogether free from these defects, it is on the whole better to have one or two good-sized bubbles than any density of texture; because it follows, that every inequality will refract pencils of light out of the direction they ought to go; and as bubbles do the same thing, but as they do not refract away so much light, they are not of much consequence.
I believe if a lens is made as thin as it safely can be, it will be quicker than a thicker one. I have two precisely the same focus, and one thinner than the other; the thinner is much the quicker of the two. An apparently indifferent lens should be tried with several kinds of apertures, till it will take sharp pictures; but if no size of aperture can make it, or a small aperture takes a very long time, it is a bad lens. M. Claudet, whose long experience in the art has given him the requisite judgment, changes the diameter of his lenses often during the day; and tries occasionally, in his excellent plan, the places of the chemical focus: by this his time is always nearly the same, and the results steady. As he is always free in communicating his knowledge, he will, I think, always explain his method when he is applied to. The inexperienced photographer is often too prone to blame his lens when the failure proceeds more from the above causes. The variation of the chemical focus during a day's work is often the cause of disappointment: though it does not affect the landscape so much as the portrait operator.
If any one has a lens, the chemical and visual focus being different, his only remedy is M. Claudet's method. And this method will also prove better than any other way at present known of ascertaining whether a lens will take a sharp picture or not. If, however, any plan could be devised for making the solar spectrum visible upon a sheet of paper inside the camera, it would reduce the question of taking sharp pictures at once into a matter of certainty.
All lenses, however, should be tried by the opticians who sell them; and if they presented a specimen of their powers to a buyer, he could see in a moment what their capabilities were.
Weld Taylor.
Bayswater.
Photography and the Microscope (Vol. vii., p. 507.).—I beg to inform your correspondents R. I. F. and J., that in Number 3. of the Quarterly Journal of Microscopical Science (Highley, Fleet Street) they will find three papers containing more or less information on the subject of their Query; and a plate, exhibiting two positive photographs from collodion negatives, in the same number, will give a good idea of what they may expect to attain in this branch of the art.
Practically, I know nothing of photography; but, from my acquaintance with the modern achromatic microscope, I venture to say that photography applied to this instrument will be of no farther use than as an assistant to the draughtsman. A reference to the plates alluded to will show how incompetent it is to produce pictures of microscopic objects: any one who has seen these objects under a good instrument will acknowledge that these specimens give but a very faint idea of what the microscope actually exhibits.
It is unfortunately the case, that the more perfect the instrument, the less adapted it is for producing photographic pictures; for, in those of the latest construction, the aperture of the object-glasses is carried to such an extreme, that the observer is obliged to keep his hand continually on the fine adjustment, in order to accommodate the focus to the different planes in which different parts of the object lie. This is the case even with so low a power as the half-inch object-glasses, those of Messrs. Powell and Lealand being of the enormous aperture of 65°; and if this is the case while looking through the instrument when this disadvantage is somewhat counteracted by the power which the eye has, to a certain degree, of adjusting itself to the object under observation, how much more inconvenient will it be found in endeavouring to focus the whole object at once on the ground glass plate, where such an accommodating power no longer exists. The smaller the aperture of the object-glasses, in reason, the better they will be adapted for photographic purposes.
Again, another peculiarity of the object-glasses of the achromatic microscope gives rise to a farther difficulty; they are over-corrected for colour, the spectrum is reversed, or the violet rays are projected beyond the red: this is in order to meet the requirements of the eye-piece. But with the photographic apparatus the eye-piece is not used, so that, after the object has been brought visually into focus in the camera, a farther adjustment is necessary, in order to focus for the actinic rays, which reside in the violet end of the spectrum. This is effected by withdrawing the object-glass a little from the object, in which operation there is no guide but experience; moreover, the amount of withdrawal differs with each object-glass.
However, the inconvenience caused by this over-chromatic correction may, I think, be remedied by the use of the achromatic condenser in the place of an object-glass; that kind of condenser, at least, which is supplied by the first microscopic makers. I cannot help thinking that this substitution will prove of some service; for, in the first place, the power of the condenser is generally equal to that of a quarter of an inch object-glass, which is perhaps the most generally useful of all the powers; and again, its aperture is, I think, not usually so great as that which an object-glass of the same power would have; and, moreover, as to correction, though it is slightly spherically under-corrected to accommodate the plate-glass under the object, yet the chromatic correction is perfect. The condenser is easily detached from its "fittings," and its application to the camera would be as simple as that of an ordinary object-glass.
However, my conviction remains that, in spite of all that perseverance and science can accomplish, it never will be in the power of the photographer to produce a picture of an object under the microscope, equally distinct in all its parts; and unless his art can effect this, I need scarcely say that his best productions can be but useful auxiliaries to the draughtsman.
I see by an advertisement that the Messrs. Highley supply everything that is necessary for the application of photography to the microscope.
H. C. K.
—— Rectory, Hereford.
In reply to your correspondent J., I would ask if he has any photographic apparatus? if so, the answer to his question "What extra apparatus is required to a first-rate microscope in order to obtain photographic microscopic pictures?" would be None; but if not, he would require a camera, or else a wooden conical body, with plate-holder, &c., besides the ordinary photographic outfit. Part III. of the Microscopical Journal, published by Highley & Son, Fleet Street, will give him all the information he requires.
φ. (p. 506.) may find a solution of his difficulties regarding the production of stereoscopic pictures, in the following considerations. The object of having two pictures is to present to each eye an image of what it sees in nature; but as the angle subtended by a line, of which the pupils of the eyes form the extremities, must differ for every distance, and for objects of varying sizes, it follows there is no absolute rule that can be laid down as the only correct one. For distant views there is in nature scarcely any stereoscopic effect; and in a photographic stereoscopic view the effect produced is not really a representation to the eye of the view itself, but of a model of such view; and the apparent size of the model will vary with the angle of incidence of the two pictures, being smaller and nearer as the angle increases. I believe Professor Wheatstone recommends for landscapes 1 in 25, or about half an inch to every foot.
Geo. Shadbolt.
Cement for Glass Baths.—In reply to numerous inquiries which have appeared in "N. & Q." relative to a good cement for making glass baths for photographic purposes, I send a recipe which I copied a year or two ago from some newspaper, and which seems likely to answer the purpose: I have not tried it myself, not being a photographer.
Caoutchouc 15 grains, chloroform 2 ounces, mastic ½ an ounce. The two first-named ingredients are to be mixed first, and after the gum is dissolved, the mastic is to be added, and the whole allowed to macerate for a week. When great elasticity is desirable, more caoutchouc may be added. This cement is perfectly transparent, and is to be applied with a brush cold.
H. C. K.
—— Rectory, Hereford.
Mr. Lyte's Mode of Printing.—All persons who have experienced disappointment in the printing of their positive pictures will feel obliged by Mr. Lyte's suggestion as to the bath; but as the preparation of the positive paper has also a great deal to say to the ultimate result, Mr. Lyte would confer an additional obligation if he gave the treatment he adopts for this.
I have observed that the negative collodion picture exercises a good deal of influence on the ultimate colour of the positive, and that different collodion negatives will give different results in this respect, when the paper and treatment with each has been precisely the same. Does this correspond with other persons' experience?
C. E. F.