494 C. Barus — Interferences of Crossed Spectra. 



speaking only those rays which lie in the common longitudinal 

 axis of rotation of the two coincident spectra symmetrical to 

 this should be in this condition. Such interference should not 

 apparently be appreciable, since the white rays are independent 

 and come from two different points of the slit. If we con- 

 sider the angular deviation of pencils of parallel rays crossing 

 ths grating, to be equivalent to the divergence of their respec- 

 tive optical axes at the collimating lens (about 45 cm. in focal 

 distance), the distance apart of two points of the slit the rays 

 of which are still able to produce interference is x = 45 X 

 A0 = 45 X 1'7 X 10- 4 =7-6xl0- 3 cm. or nearly -lmm. Hence 

 points of white light in the slit about - 1 mm. apart along its 

 length produce the band of interferences in question, extend- 

 ing in colored light from red to violet. 



5. Interference of the corresponding first order spectra of 

 the grating, in the absence of rotation. — This apparatus seemed 

 to be of special interest, since the rays used do not retrace their 

 path and are thus available for experiments in which rays trav- 

 eling in one direction only, are needed.* I have tried both the 

 adjustments given in figure 4a and b. The latter, since the 

 rays are more nearly normally reflected at the mirrors Jf and 

 JY, has some advantages ; but the other succeeds nearly as well. 

 The difficulty encountered is a curious one of adjustment, 

 which was not anticipated. In other words, if the longitudinal 

 axes of two identical spectra are in coincidence, the Frauen- 

 hot'er lines are likely to be at a small angle to each other and 

 complete interference is therefore impossible. Again if the 

 spectrum lines are in coincidence, the longitudinal axes usually 

 diverge by a small angle. Furthermore the interferences are 

 almost always eccentric and the lines hairlike, indicating distant 

 centers. I have not succeeded in making a perfect adjustment, 

 systematically, but the discrepancies are themselves interesting 

 in their bearing on the subject of this paper. In figure 4, L 

 is a vertical blade of white light from a collimator with fine 

 slit and G is the grating. The two first order spectra leaving 

 the ruled face at the line g strike the opaque mirrors M and 

 JV, the former on a micrometer moving the mirror parallel to 

 itself. From M and JV the rays reach the half silvered plate 

 of glass, ITS, where one is transmitted and the other reflected 

 into the telescope T. The coincident rays R are superfluous. 



After placing the parts and roughly adjusting them for sym- 

 metry with sunlight, the finer adjustment may be undertaken. 

 It may be noticed that the two systems M and N, and G as 

 well as IIS, can be used for further adjustment, separately. 

 All are provided with adjustment screws relatively to rectan- 



*Cf. this Journal, xxxiv, pp. 101, 1912, on an air column carrying elec- 

 trical current. 



