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Ill THE PHOTOGEAPHIC NEWS. LFEBRUANY 20, 1885. combination in which an achromatic concave lengthens the focus of a shorter achromatic convex ; for though it had behaved excellently well with views even up to 41 by 27 inches, yet a much smaller copy was a severer test. And having by me a convex achromatic lens of the usual shallow meniscus form of old telescopic objectives, and of 82 inches focus, I took off the front lens of my Ross’s 4A (39 inches focus as single lens) and substituted thah The definition was bad until much stopped down, the aberra tion being over-corrected ; but, and probably partly from that cause, I found the field perceptibly concave when quite a small stop was used, and a far better marginal definition than with the modified orthoscopic. After that I arranged another back combination of odd lenses, finding the best position for definition to be with the concave next the plate, as when I had used the doublet front lens. The definition is excellent with a properly-shaped convex out of the Ross ; the distances and focal lengths also giving correct achromatism. Wishing, however, to keep the complete lenses separate, I substituted an equi-convex, which required a stop, and this being also over-corrected for dispersion, unless toomuch separated forthe aberration; I have placed a common 21 inch p'ano-convex lens of 11} feet focus just in front of the stop ; and now the lens is an excellent copying combination of 37 inches equivalent focus, sensibly free from distortion, coincident in foci, and having no perceptible convexity of field when used to cover a 24 inch plate with a stop of one inch, the marginal definition being also very good. Now all these are odd lenses, the cost reckoned almost by shillings, and the fitting only in a wooden box. Even the front lens was made up by joining a 5 inch convex with a 44 inch con cave lens, cemented with castor oil (a hint from Captain Abney in your paper) and fitted in a wooden cell. Cannot Petzval combinations of 4, 6, or 10 feet focus, be made to copy paintings, &c., cheaper and better than the shorter focus lenses usually used, and as large as can be wished ? To further illustrate the inter-dependence of lenses in form and position. If I had separated the convex and concave components of that back combination which I found to give non-coincidence, by about 2 inches, that error would have been corrected, and coincidence of foci ob tained, because the pencil emerging from the convex has a form convergent to a focus some 21 inches distant. The 2-inch interval would reduce this to 19 at incidence on the flint concave, and the dispersion, being as the area of the circle corresponding to the pencil, would be reduced from the square of 21 to the square of 19, or from 11 to 9. It comes to nearly the same thing as regards dispersion, whether the correction be thus reduced from 11 to 9, or thepower of the convex increased from 9 toll ; and adding the central convex lens was equivalent to this. But it is not an equivalent as regards aberrations, either direct or oblique. A shorter focus equi-convex would have had more positive aberration. To give it a shape more convex towards the stop, and less so towards the flint 1-ns, might have reduced that sufficiently, but such a shape increases the positive oblique aberration, reducing the flatness of the field. Perhaps it would not otherwise injure marginal definition, because though positive confusion and astigma tism are increased, that increase may merely correct that of the other lens, whose negative oblique aberration may probably leave a balance of other negative error while flattening the field. A maker would probably change the forms and positions of both lenses to the usual type, and perhaps slightly under-correct the front lens. But there is another plan at which he might shake his head. If for the one convex lens of the back combination two thin ones were substituted, the sura of their powers being equal, both direct and oblique positive aberrations would ue diminished, and a more favourable balance of both aber rations and dispersions obtained at the cost of two more reflecting surfaces ; and, for a large copying lens, I should think this the best plan. Any loss of light would be fully compensated by the larger stop then admissible, and the evil of stray reflected light, reduced by a dark margin round the picture to be copied, and a rectangular aper ture in front of the lens, admitting light only to the rec tangular plate, would be insensible. I have also noticed, when putting the deepest concave surface next to the plate, a remarkable freedom from flare, which seems a probable advantage of that position. We can now better understand something of the diffi culty and complexity of calculations for finding the best forms and positions uniting all compatible advantages. For experiments and some intelligent appreciation, a general understanding is enough. Not so to deduce work- ing formula?. There is yet one other consideration. Ordinary lenses are corrected for rays less divergent than is usual in copy ing ; and the condition that, with the best correction for slightly divergent rays should be joined one as perfect as possible for rays much more divergent, is one of some im portance. Sir John Herschel used this condition in his general solution of the best forms for telescopic objectives, and it led him to a form very unfavourable to a flat field. I am inclined to doubt whether any general practical form can be indicated as best fulfilling this condition, but I think I may say certainly that a division of the refraction among many surfaces is distinctly favourable, and such ex perience as I have had in comparing Petzval lenses with aplanatic doublets in copying to the same size tends to confirm that opinion. ARTISTS VERS US PHOTOGRAPHS. BY MB. PAE.* THE works which artists produce and those which photographers produce are two entirely different things, and should, I think, never clash together. The art work belongs to the region of emotional thought, and the photograph to that of science ; the more emotional thought there is evidenced in a picture the more is it a work of art; whereas, the better the photograph, the more the evidence of scientific knowledge of the instrument and chemicals used. All nature appears before the artist, and what ever he sees he can depict and give us his thoughts about them; but it is very different with the photographer: he cannot, if he wishes, with the instrument he uses, depict many phases of nature; he is limited within certain boundaries. The grandest and most beautiful aspects af nature are utterly out of reach of photography ; the lower strata of rain cloud, when they are in forcible light and shade, can be photographed, but all the other aspects of sky—and they are countless, such as sunsets, sunrises, mountain-like clouds and cirrus, in all their variety of form and relative depth and shade, to the depth of open sky—is a dead blank to the photograph ; and although in mountain scenery the mist covering out part of their form can be photographed, yet the grandest effects of cloud and mountain in their relative depths of shade to each other and to the nearer landscape cannot be recorded, for when nature is using her full scale of light and shade, compared wih the light and shade at the photographer’s command, I sha'l be very moderate if I put it as 200 to 10 ; sn, on this account, if the photographer goes for the shadows, he sneriees and loses his sky and light; if he goes for the light, he sacrifices and loses his shades ; he has to choose between these two evils, he cannot go for all ; but the artist has all at his com- maud ; he can arrange hie limited scale just as he pleases in order to give the relative depths of the masses of light and shade, from the deepest shadow to the highest light, and in doing this he give; us his thoughts, and leads us to those parts in nature which he wants us to rest most on. The photographer can think what he likes, and can be impressed with the scene he is photo- graphing in a thousand different ways, but thelens takes no notice of his impressions or thoughts, but casts the scene on to the plate according to the law which governs light and lenses. And this brings us to the question, does the lens give correct drawing ? Does it represent objects in right relative size to each other ? If we take a photograph and compare it with a tracing on glass of the same subject, the same size and focus, we shill find that the lines of the photograph do not exactly coincide with the tracing ; there is a variation from the truth in the • A communication to the Newcastle-on-Tyde and Northern Cvunties Photographic Association.
