REVERSED AND NON-REVERSED SPECTRA. 21 



of the grating is so cut up by the strong, stationary interferences that it is 

 unavailable. The grating plate must, of course, be slightly wedge-shaped, 

 otherwise all the spectra would be superposed. In case the ruled face is 

 half-silvered, however, the stationary interferences are practically absent, 

 while two strong spectra are reflected from the silvered side. The phenome- 

 non may then be produced at all distances of G from M and N (2 meters and 

 less), but best at distances within i meter. It is, however, frequently 

 hard to find unless different distances apart of the mean D lines are tested. 

 This may be due to the fact that the silver film is not quite equally thick. 



Besides the symmetrical position, gT, figure 12, the two corresponding 

 unsymmetrical positions g'T' were tested with success; and it appeared that 

 while in the case gT the phenomenon is virtually linear, dark or bright, like 

 a Fraunhofer line, a succession of dark lines inclined to the vertical may 

 appear for the unsymmetrical position g' T . Dark lines are apt to be broadened. 



Questions relative to the effect of oblique incidence were also tested by 

 aid of the concave-mirror method shown in figure 1 1 , the white light from C 

 to G being conducted in an inch tube of pasteboard, immediately under the 

 concave mirror, m. Figure 13,0, shows the general disposition of apparatus. 



The angle of incidence i is gradually increased, until the return rays from N 

 meet the grating at nearly grazing incidence. No essential difference in the 

 phenomenon was observed, however, except that it was apt to be broader 

 in the non-symmetrical positions and to suggest fine new lines in parallel 

 with the old. In a return to the symmetrical position, sharp lines were 

 especially distinct, usually showing one dark and two bright lines, while two 

 dark and one bright occurred less frequently. It could be seen quite vividly 

 with the naked eye. When the telescope was used and the ocular drawn 

 far forward, the multilinear form was often suggested. On broadening the 

 slit the black lines vanish first and a flickering band remains after the Fraun- 

 hofer lines are gone. Finally, the phenomenon could be seen even when the 

 longitudinal axes of the spectra were not quite coincident, but it rapidly 

 became fainter in intensity. 



Figure 13, &, suggests a method of using a reflecting grating, either plane 

 or (possibly, if the incident light is parallel) concave, for the production of 

 the phenomenon. G is the grating, receiving the collimated white light, L, 

 which is diffracted toward M and N, thence reflected (at a different elevation) 

 back to G, to be again diffracted towards T, above or below the direct beam, 

 where it is observed. I have not, however, been able to obtain results with 

 these methods owing to subsidiary difficulties. 



