EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 101 



larly reflected from the ruled surface than is transmitted, and more reddish 

 light transmitted than is reflected. Since the plate grating is not quite plane 

 parallel, two of the four rays, b' and y', are seen in the same colors in the 

 telescope. This is a great convenience in adjusting the displacement inter- 

 ferometer, where the spectra from b alone are wanted, and the y ray may be 

 screened off at N, while the other y' has no spectrum. 



The transmitted rays / after reflection show very little difference, the one 

 reflected at g being perhaps slightly yellowish as compared with the other. 



The spectra from b and y, if compared one 

 above the other, are practically identical. 

 The difference is not sufficiently marked to 

 be discerned by the eye. Multiple reflection 

 from the two faces gave no further results. 



Finally, to be colored blue, the beam must 

 be reflected from the air side and not from 

 the glass side, where but little appreciable 

 effect is produced. If the grating is turned 

 1 80, both the b and y rays are nearly white, 

 while the / rays now correspond to the b and 

 y rays and are vividly colored. 



Outside the ruled surface and with any or- 

 dinary unruled plate of glass, all images are 

 of course white. I mention this merely since c/l( 

 one might suppose the absorption or color of 

 the glass to have something to do with the 

 experiment. The film grating, where sharp reflection takes place from the glass 

 and not appreciably from the film, does not ordinarily show the phenomenon ; 

 but in case of the single-plate film grating of paragraph 60, it is astonishingly 

 strong in the refracted slit images seen in the telescope. These are, in fact, 

 azure blue when coming from the mirror N and reflected from the front side 

 (toward the lamp) of the film ; deep brown when reflected from the rear side, 

 after having passed through the film. The two images may be superposed by 

 rotating M with the production of nearly white light. Moreover, the marginal 

 light (otherwise identical but not passing through the film) is white. The 

 images in question are sharp, but it is possible that the material of the film 

 may somewhat contribute to the color. 



65. Explanation. Scattering is usually and perhaps essentially associated 

 with diffuse reflection. The present phenomenon, however, is strictly regular 

 reflection; i.e., there is a wave-front, for the blue and yellow slit images are 

 absolutely sharp in the telescope. This is the interesting feature of the phe- 

 nomenon, which associates it at once with the recent famous discovery of 

 Friedrich, Knipping, and Laue relative to the reflection of X-rays from the 

 molecular planes of crystals, and it is for this reason that I call attention 

 to it. 





