The advance of photography : its history and modern applications (1911)

322 THE ADVANCE OF PHOTOGRAPHY defined picture may be produced on the screen. The tube B C contains the lenses which concentrate the light. It must be remembered that in enlarging the image as is here done, the actual amount of light received on any particular part of it will be very greatly diminished. In fact, if it is enlarged three times, the light on any particular part is diminished to one-ninth its original value ; if enlarged fourfold, to one-sixteenth, and so on, the light received being inversely proportional to the square of the linear magnification. With such a diminution of brightness it would be extremely difficult to distinguish anything by the unaided eye, if care were not taken to throw a very strong beam of light upon the object. The system of lenses in^C answers this purpose, concentrating on the microscopic object the sun's rays which are reflected by the mirror M into the tube B. When this apparatus was made use of for obtaining microphotographs, the room into which the image was projected had to be completely dark, so all that was necessary was to expose the photographic plate in the exact position in which the image on the screen appeared well defined. Such a piece of apparatus as the above, presented too large a number of difficulties to be surmounted ever to become very generally used, the chief perhaps being that a special room was required for its use. The Microscope. — The ordinary microscope can, however, be used in a much simpler manner, and with a little practice good results are obtained. For this purpose it is essential that the stand should be perfectly rigid, of a large type, and provided with a rotating and centering stage ; and further, a very good fine adjustment should be possible, so that perfectly well defined images can be produced These properties are possessed by such instruments as shown in fig. 134. To produce photographs with the help of such a