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660 History of Three-Color Photography
a pinhole camera might be used or the lens considerably stopped down, to bring the grating and landscape into common focus on the plate. The apparatus might also be used to make interferential color photographs if the sensitive film were backed with a mercury mirror. The colored picture was reconstituted by placing a positive in the same plane that the negative originally occupied, the grating being illuminated by white light. Or a lamp and condenser might be placed behind the transparency and the image thrown on a screen. In another form the image was first projected on to the grating by a supplementary lens and condenser.
Lanchester gave the following rough sketch, Fig. 189, and explanation: let A be an objective producing the image not more than two inches in diameter in the plane M. Let G be a grating of 300 lines to the inch, situate in the plane M. The opaque inter-spaces of the grating to be twice the width of the clear spaces. Let B be a second objective, projecting an image of the plane M on the ground-glass screen S, with a magnification of two diameters. Let P be a narrow-angled prism, placed just in front of the grating, of such dispersive power that the spectra which it produces on the screen S just lie next to one another without overlapping. Provided the image at M were white (or, in other words, only white light fell upon the grating), then the effect of the above arrangement would be to cover an area of four diameters on the screen with a series of contiguous spectra, 150 per inch. To the unaided eye this surface would appear of a greyish white. In the diagram, part of the screen is shown covered with these spectra, of an exaggerated width. Now suppose for a moment that another grating of 150 lines per inch were laid over the screen, the lines being parallel to the spectra. Then each line of this grating would cover up the same portion of each successive spectrum and the whole field would appear to the eye of some uniform color. If, for example, the lines covered up the red portion of each spectrum, then the field would appear in the complementary color to red, viz., green. If the number of lines of the grating laid over the screen do not exactly correspond with the number of spectra, a comparatively quite long spectrum can be visually produced, because then the successive black lines do not block out exactly the same portions of spectra, but successive portions.
In a private letter to Rheinberg, who later took up this subject, Lanchester stated8 that his apparatus consisted of a camera and a steel tubular frame, attached to its front, this frame carrying at its front end a silvered glass screen, with slits scratched through the silver, twenty to the inch ; the slits forming about one-quarter of its area. The ratio of the reduction was 5:1, the screen being 15 inches square, and the plate 3j4 inches square. The angle of the prism was about 2 to 3 degrees ; the pitch of the spectra on the plate was 100 per inch, which were sufficiently fine to fuse optically. The object of the silvered mirror was to reflect light to the subject, which had to be placed as near the screen as possible.