C. Barus — Interference of Reversed Spectra. 4] 5 



horizontal axis, in order that the focus F may be appropriately 

 placed between M and JV: with G at one meter and m at two 

 meters from F, the disposition is good. The micrometer M is 

 easily at hand. Though the direct beam may be screened off, 

 the glare reflected back from the grating and the glare from 

 the objective of the collimator are not excluded, as stated. In 

 fact it was eventually found necessary to carry this pencil in an 

 opaque tube reaching from the objective of the collimator, as 

 far as the grating. 



Second order spectra are too faint and cannot be seen, unless 

 the fflare is excluded in the manner stated. 



An alternative method of half silvering the ruled face of the 

 grating and then using it as a reflector, was tried with success. 

 The beam of parallel rays from the collimator L, fig. 2, are 

 transmitted by the grating (ruled, half-silvered face, g toward 

 the mirrors M and JV) and the two diffracted beams then 

 returned by the opaque mirrors M and JV, to be in turn diffracted 

 by reflection, into the telescope T. The stationary interfer- 

 ences are practically absent, while two strong spectra are 

 reflected from the silvered side. The phenomenon may then 

 be produced at all distances of G from M and iV(two meters 

 and less), but best at distances within one meter. 



Besides the symmetrical position, gT, fig. 2, the two cor- 

 responding unsymmetrical positions, g' ' T ', were tested with suc- 

 cess, but 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 

 fig. 1, the white light from (7 to G being conducted in an inch 

 tube of pasteboard, immediately under the concave mirror m. 

 Figure 3 gives the general disposition of apparatus. The angle 

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

 JV 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. On 

 broadening the slit the black lines vanish first and a flickering 

 band remains after the Fraunhofer 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 4 suggests a method of using a reflecting grating, 

 either plane or (possibly, if the incident light is parallel) con- 

 cave, for the production of the phenomenon. G is the grating, 

 receiving the collimated white light, L, which is diffracted 

 toward M and JV, 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, 



Am. Jotjr. Sci. — Fourth Series, Vol. XLI, No. 245. — May, 1916. 

 29 



