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.
PHOTOGRAPHY IN NATURAL COLOURS 245
(for this purpose 2000-3000 to an inch) at equal distances apart. The principle on which Wood's process depends is described by him as follows : " If a diffraction grating of moderate dispersion and a lens be placed in the path of a beam of light coming from a linear source, and the eye be placed in any one of the spectra formed to the right or left of the central image, the entire surface of the grating will appear illuminated with light of a colour depending on the part of the spectrum in which the eye is placed. If one part of the grating has a different spacing from the rest, the spectrum formed by this part will be displaced relatively to the first ; and if the eye be placed in the overlapping part of the two spectra, the corresponding portions of the grating will appear illuminated in different colours."
In a simple way this may be explained by remembering that the amount of diffraction depends upon the closeness of the lines, and by using gratings of different rulings it can be arranged so that any two colours may be made to overlap, since, e.g., red will be diffracted by one grating just to the same extent as green by the other. When such a thing as this occurs with red and green colours, the result will be that the eye will receive yellow light, while if, in addition, a third grating is used by which blue violet light is diffracted to the same extent as red and green respectively by the other two, and then all three spectra are made to overlap, the light received by the eye will be white. Again, if different parts of a picture are represented by lines of a fineness corresponding to that required to show a given colour, that colour will be visible in the picture when viewed from a suitable standpoint.
In practice Wood took three negatives, using red, green, and blue screens respectively. From these negatives three positives are made on ordinary lantern slides ; and in this connection it is important to remember that albumen-coated plates must be used, since the warm water used in subsequent development would dissolve a gelatine