We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
Prismatic Dispersion and Allied Processes
663
camera. Subsequently Cheron devised another apparatus14 in which he used a 10-inch focus lens to project the line screen on to the plate, Fig. 190, but the inordinate length of this apparatus — 42 inches for a 2^-inch plate — made it extremely cumbersome.
M. Raymond15 also described an apparatus, Fig. 191, consisting of a long camera with lens of large aperture, which formed an image a' V on a screen T, of parallel horizontal lines, the opaque ones being twice the width of the transparent and a few fractions of a millimeter wide. The image was then formed, after leaving T, of bright and dark lines, the latter having the color of the object in front of L. This image a' b' was vertical and was transformed after passing through a second lens placed at twice its focus from the screen T into an image, which might be received on a ground glass at the end of the apparatus. If the prism P were placed in the path of the rays, and it was so chosen that its dispersion was equal to one of the transparent lines plus an opaque one, the image b' a" would be the spectral image of a! bf, the spectra being side by side and not overlapping, thanks to the opaque lines. Thus in Fig. 192
XT
V.
JJP — —
. — ^Z^T^z^z
M0
Fig. 191. Raymond's Prismatic Dispersion Apparatus.
the spectra d' e' , f g', h' i' correspond to the lines d e,f g, h i, and cover the image of the opaque lines. If the ground glass be replaced by a colorsensitive plate, it would be possible to photograph these spectra, and the exposure would only be about three times longer than in the ordinary way, and this would be due to the absorption by the opaque lines. Subsequently Raymond suggested a still simpler arrangement, which, however, falls into the next section.
F. Urban16 proposed to first break up the image by means of a cross line or other screen, then pass it through a dispersing medium, such as a simple prism, a direct vision prism, combinations of prisms, small particles of color-diffusing material or a diffraction grating. This is obviously no more than a modification of the previous methods.
The most perfectly worked out system was that by J. and E. Rheinberg,17 but it still remains more of a laboratory than a practical process. A specially computed direct-vision prism was used, which gave nearly a normal spectrum. The line screen had 500 lines to the inch with the clear