REVERSED AND NON-REVERSED SPECTRA. 109 



distances NG (face toward the light) and MG are equal, the interferences 

 are then easily found by moving the mirror M on the micrometer toward 

 the grating. 



As compared with the other non-linear interferometers used under like 

 conditions, the present instrument, even when mounted on a y^-inch gas- 

 pipe, RR, showed itself remarkably steady, so that rings could be observed 

 in spite of the tremors of the hill on which the laboratory is built. 



59. Film=grating adjustment. Michelson's interferometer. If the grat- 

 ing G is a film grating, like those in the market, with 14,000 lines to the inch, 

 it should be mounted smoothly on the unruled side, on a thick glass plate, 

 with Canada balsam, and without a cover plate for the ruled side. It is to be 

 adjusted with the glass side toward the source of light, so that the reflection 

 taken may be from this side only (see r, fig. 78). In the telescope, T, directed 

 toward the reflected beams, two slits (one for each component beam) only 

 appear, as the glass plate does not reflect on the side covered by the grating 

 (g in fig. 78). The slits placed in coincidence will then show the elliptic inter- 



80 78 



ferences in the diffracted beam D at the proper distances. With so large a 

 dispersion as the above, the ellipses are usually too large. They should then 

 be reduced in size by a compensator placed in the beam on the ruled side of 

 the grating; or, preferably, the grating may be mounted on a plate of glass 

 fully i cm. (or more) thick, as in figure 78. This thick plate has the additional 

 advantage of eliminating the stationary interferences due to the front and 

 rear faces of the grating. In case of thin glass plates (2 or 3 mm.), these 

 stationary interferences are very strong, coarse, vertical lines and exceed- 

 ingly annoying. 



If the film grating is carefully mounted in this way, it is nearly as good as 

 a ruled grating. There is, however, one insuperable objection, inasmuch as 

 the ruled face, though it does not reflect sharply, does diffract, and this more 

 strongly than the other. Thus there are always 3 superposed spectra in 

 the telescope, the third coming from the film side only, whereas the other two 

 are produced by the rays coming coincidently from r on the unruled side of 

 the grating. Hence the velvety blackness of the interferences in case of the 

 ruled gratings can not be reproduced by the film grating, since the interfer- 

 ences are spread out on a colored ground. They are, however, quite strong 

 enough for all practical purposes, and the lines are sharply and symmetrically 



