MICRO-PHOTOGRAPHY. Under this head may be included two different processes. One is of little or no practical utility, and con sists in copying objects on an exceedingly small scale, the photograph being intended to be viewed tlrough a magnifier, or microscope. The other, which is a branch of photography of the highest interest and importance, consists in producing enlarged photographs of minute objects—that is, in fixing the images obtained in the microscope.
With respect to the former of these two processes, a few words will suffice. In order to obtain an extremely minute image of an object, it must either be placed at a much greater distance than usual from the lens, or a lens of very short focus must be used. Any of the' ordinary forms of photographic lens may be made of one inch focal length, and then by placing the object or group to be copied at a suitable distance, a photograph may be obtained in a drop of collodion. The only difficulty in the process is that of focussing. This may be done on the sensitive film itself, a yellow glass being interposed between it and the lens while focussing, and a powerful magmfier employed to examine the focus. Negatives may, of course, be copied either by transmitted natural or artificial light. Enough has now been said about a process which must strike any reasonable person as somewhat trifling and childish, when he considers how many valuable applications of photography remain yet to be worked out.
The copying of magnified images in the microscope, in such a way as to satisfy sound criticism, is a matter of much greater difficulty. The pririciple of the thing is simple enough, and will be understood by reference to Fig. 6, page 63, in which P Q may be considered a microscopic object, and p q its magnified image ; but the difficulty consists in obtaining a flat field when the ratio between the size of the image and the object is considerable. In the low-power lens of an ordinary microscope this ratio is as 8 : 1; and the size of the field about 1.4 inches. By using a lens of double
the focal length, and removing the object to double the distance from it, a picture of the same size is obtained, the lateral pencils have less obliquity, and the field is much flatter, but the amount of light is reduced to one fourth. It is evident, therefore, that the optical difficulties are diminished by increasing the intensity of the light. It would be well, therefore, to perform micro-photographic operations by sunshine transmitted through the object by means of a reflector and condenser; or else with the Bude light. A great amount of solar light is, however, accompanied with great heat, and a difficulty is introduced from this cause. The photographer has, however, two other resources. He may either work upon plates which are ground spherical, like a watch glass ; or the object itself may be inclosed between curved glasses. The former plan would probably be found the best.
With respect to the best form of lens. There is an erronous im pression afloat among amateur microscopists who know but little of optics, that the obliquity of the pencils when high powers are used is something considerable, and that modem science has brought under its control the various vagaries of oblique pencils. This is not the case. The vagaries of oblique pencils are incurable, and like. the quadrature of the circle they set at defiance all the resources of the mathematician. The truth is, the obliquity of the lateral pencils in the microscope is inconsiderable compared with that of the lateral pencils in an ordinary camera obscura. The best form of lens for obtaining micro-photographs is therefore an achromatic single lens, as represented in Fig. 6, and constructed on a similar principle to the object glass of the telescope. By increasing the number of lenses, light is lost, and distortion introduced.
The principles of this branch of photography having been now, it is hoped, clearly explained, the various modes of applying them may be left to the ingenuity of the reader.