EFFECT OF FINENESS OF RULING.
The extent of the diffraction and the distance of the spectra from the image of the central slit depend upon the fine- i! ness with which the screen is ruled, which fact is turned to advantage, as will now be shown. If a small hole were made in the r lantern screen, just where the violet por tion of the spectrum was visible, and the eye were applied to the aperture behind, looking towards the illuminated grating, the latter would appear entirely violet. Or, if the hole were made instead in the red part of the spectrum, and looked through as before, the grating would seem to be all red. Now, as before explained, 1 since the amount of diffraction and the position of the spectra in relation to the central slit vary with the closeness of in principle from any of those previously mentioned. A diffraction grating, it should be explained, consists of a sheet of glass or metal ruled with a very large number of Pine parallel lines, at equal dis tances from each other, and has the operty of splitting up white light into its constituents, very similarly to a glass prism. If a lantern, A (Fig. 578), is used to throw a beam of light through an opaque slide having a narrow slit or clear space of glass, an image of the slit will appear on the screen at B. If, now, a
glass diffraction grating, r, is placed between the lens and the screen, the white the grating, it is evident that if the first grating were to be removed, and one with the lines ruled closer together or farther apart were substituted, the eye applied to the opening in the screen would no longer see the same colour as before, but a different one, because the spectrum would have shifted its place on the screen. For instance, with gratings ruled with 2,000, 2,400, and 2,750 lines to the inch, as used by Professor Wood, if the hole in the screen is made in the red portion of the spectrum obtained with the coarsest grat ing, the green rays will conic over the opening if the intermediate grating is used, and the violet would be seen on replacing it with the finest ruling.
will actually be seen when such a picture is looked at from the correct point of view. It is on this principle that the pro cess is founded. Three negatives are first taken through red, green, and blue-violet screens. Suppose the subject is such as that shown by Fig. 579, which represents