There are two modes by which this very serious evil may be remedied. One is objectionable, as it involves an error in principle. It consists in placing a stop midway between the lenses, and thus preventing the reflected rays from the inside of the tube from entering the camera. But this plan not only cuts off a part of every oblique pencil, but it compels the part that is available to pass through the outside of the front lens, whereas every pencil should pass centrically through that lens. The other plan is correct in principle, and will be understood by referring to Fig. 4.
In this arrangement of the lenses, the posterior lens is larger than the anterior, and they are not mounted in a tube, but in wooden partitions ; no reflected light can, therefore, by any pos sibility enter, and none of the rays from the oblique pencils are cut off. Every pencil passes centrically through the front lens, and excentrically through the posterior lens, as it ought in order to get the maximum flatness of field.
But even this arrangement is not sufficient to produce a cle,ar picture when the lenses are exposed to a strong hght, the bright part of the sky, for instance. Diffused light exists in the atmo sphere, and if we wish to see distant objects clearly, we must look at them through a tube. The stars, for instance, are visible at noon-day from the bottom of a well. Bearing this principle in mind, an important addition should be made to every camera, in the shape of a long darknened tube in front, as shown in Fig. 5, which is a section of a camera, with all the proposed improvements introduced.
A camera constructed on this principle would be equally suitable for views or portraits, because its lens would give as flat a field as an ordinary view lens, and splendid definition might be obtained by using a stop immediately in front of the front lens. By removing the stop, and working in a strong light with a full aperture, it would be suitable for taking instantaneous pictures. It was with a lens of this kind that M. Flacheron produced his magnificent views of Rome some years ago, and it is surprising that this form of lens should have gone out of fashion, and been supplanted by one in which everything is sacrificed to the central pencils. The gre,at merit of a photograph is to be equally good in every part, and not to have the edges of the field sacrificed to the centre, a defect which, in views of landscapes or architecture, is perfectly unbearable.
Should it be necessary, when using the above form of camera, to turn its axis upwards, the slide must be always kept stiictly vertical by means of the arrangement for that purpose, otherwise, all the vertical lines would appear to have a vanishing point in the zenith.
The camera and slide should be square, and the plates large enough to take in the whole of the circular picture that is formed by the lens. Circular views are extremely natural and pleasing, particu larly when softened off into darkness at the edges. It is a circular picture which is always formed on the retina, and a view bounded by straight lines at right angles always conveys to the mind the idea of looking at things through a rectangular hole in a box, and inter Peres with the idea of space and freedom of range. There is cer tainly something unnatural and disagreeable about it. Large circular pictures are remarkably fine when viewed in the reflecting stereoscope.
Copyiv Cassera.—This camera is used for obtaining copies of photographs or prints, either by transmitted or reflected light, of a different size from the original. When the copy is to be less than one half the linear dimensions of the original, the ordinary camera, with a portrait or view lens, may be employed ; but when the copy is to be nearly as large, or even larger, than the original, some modifications must be introduced in the form of the ordinary camera, and also in the lens.
The principle to be borne in mind in copying an object on a dif ferent scale is this—that the linear dimension of the copy bears to the corresponding linear dimension of the original the same ratio that the distance of the copy bears to the distance of the original from the lens. For instance, let C be a certain linear dimension of the copy, and 0 the corresponding linear dimension of the original, and let U be the distance of the original from the lens, and V the equivalent focal length of the lens corresponding to the distance U ; then C : 0 : : V : U.
Hence we arrive at the following important conclusion, viz., that so long as the ratio V : U remains constant, U may be increased as much as you please. Now, as the principal defects of photographic lenses proceed from the obliquity of the lateral pencils, and as the obliquity of these pencils is diminished in proportion as U is in creased, it is evident that the lens should not be brought so near to the object to be copied as to introduce very oblique pencils, but that it should be placed at a distance from it equal to at least three times the longest dimension of the object to be copied, and be of sufficient length of focus to give an image of the required size.