860 



SCIENCE. 



[N. S. Vol. IX. No. 234. 



interfering with the light furnished by the 

 first structure. This cannot, of course, be 

 done with inks, since if we print green ink 

 over red the result will not be a mixture 

 of red light and green light, but almost 

 perfect absence of any light whatever ; in 

 other words, instead of getting yellow, we 

 get black. Let us consider, first, how a 

 picture in color might be produced by dif- 

 fraction. Place a diffraction grating (which 

 is merely a glass plate with fine lines ruled 

 on its surface) before a lens, and allow the 

 light of a lamp to fall upon it. There will 

 be formed, on a sheet of paper placed in the 

 focal plane of the lens, an image of the 

 lamp flame, and spectra, or rainbow-colored 

 bands, on each side of it. Now, make a 

 small hole in the sheet of paper in the red 

 part of one of these spectra. This hole is 

 receiving red light from the whole surface 

 of the grating ; consequently, if we get be- 

 hind the paper and look through the hole 

 we shall see the grating illuminated in pure 

 red light over its whole extent. This is 

 indicated in Fig. 1, where we have the red 



CRATING LENS 



little lower down in the diagram, and, in- 

 stead of the red falling on the hole, there 

 will be green ; consequently, if we look 

 through the hole we shall see this grating 

 illuminated in green light. A still finer 

 ruling will give us a grating which will ap- 

 pear blue. ISTow, suppose that the two first 

 gratings be put in front of the lens together, 

 overlapping, as shown in Fig. 2. This 



end of the spectrum falling on the hole, the 

 paths of the red rays from the grating to 

 the eye being indicated by dotted lines. 

 Now, the position of the spectra, with 

 reference to the central image of the flame, 

 depends on the number of lines to the inch 

 with which the grating is ruled. The finer 

 the ruling the farther removed from the 

 central image are the colored bands re- 

 moved. Suppose, now, we remove the 

 grating in Fig. 1, and substitute for it one 

 with closer ruling. The spectrum will be a 



combination will form two overlapping 

 spectra, the red of the one falling in the 

 same place as the green of the other, 

 namely, on the eye-hole. The upper strip, 

 where we have the close ruling, sends green 

 light to the eye and appears green ; 

 the under strip, with the coarser 

 ruling, sends red light to the eye 

 and appears red ; while the middle 

 portion, where we have both rul- 

 ings, sends both red and green 

 light to the eye, and in conse- 

 quence appears yellow, since the simulta- 

 neous action of red and green light on any 

 portion of the i-etina causes the sensation of 

 yellow. In other words, we have, in super- 

 posed diffraction gratings, a structure capa- 

 ble of sending several colors at once to the 

 eye. 



If we add the third grating we shall see 

 the portion where all three overlap illumi- 

 nated in white, produced by the mixture 

 of red, green and blue light. 



Three gratings, with 2,000 lines, 2,400 

 lines and 2,7.50 lines to the inch, will send 

 red, green and blue light in the same di- 



