496 0. Barus — Interfei^ences of Grossed Spectra. 



paragraphs 2 and 3. Nevertheless I have looked upon these 

 results as additional proof of the possibility of interference. 

 For in neither case ought they to occur if the spectra are not 

 quite coincident horizontally and vertically. If they do occur, 

 it would seem that a certain small latitude of wave length 

 adjustment is permitted even with light waves. 



The cause of this lack of simultaneous parallelism I was at 

 first inclined to refer to the grating itself, as it occurred with 

 an Ames grating ruled on glass, with a Michelson reflecting 

 grating and with a film grating, in about the same measure. But 

 subsequently, on adopting the method of figure 4, b, the diver- 

 gence was largely removed and the interferences were now vis- 

 ible throughout the whole of the spectrum. The discrepancy is 

 probably due to insufficient normality of the plate of the grat- 

 ing to the incident white ray, since one of the rays is twice 

 reflected. In any case the adjustment of the coincident sodium 

 lines must be very accurate if the fringes are to be sharp ; cer- 

 tainly as little as half their distance apart will obscure the phe- 

 nomenon. 



Though the spectra are bright the interferences are not as 

 good as with the usual method (paragraph 1) ; i. e. the dark lines 

 are not black. Neither have I found an available or systematic 

 method for centering the fringes, so that the lines obtained are 

 usually delicate. Again the position of the collimator, both as 

 regards slit and lens, is here of very serious importance. Any 

 micrometric horizontal motion of either in its own plane will 

 throw the fringes out. Finally the whole spectrum travels 

 with the motion of the micrometer mirror M. The apparatus 

 is thus too difficult to adjust for use, to be of practical interest 

 when simpler methods are at hand. The effect of tremors act- 

 ing prejudicially on so many parts is exaggerated. 



6. Conclusion. The phenomena of paragraphs 2, 3 and 4, 

 showing definite and characteristic interference in case of two 

 coincident spectra, crossed either on a longitudinal or transverse 

 axis, represent the chief import of the present paper. These 

 results cannot be due to the diffraction of a slit (regarding the 

 line of coincidence as such), owing to their relatively small 

 magnitudes and their independence of the breadth of the slit. 

 Since there is in each case but a single line of points or axis, 

 the disturbance of which comes from identical sources, we 

 might regard the image of this line in the telescope to be mod- 

 ified by the diffraction of its objective. But if the interfer- 

 ences originated in this way, the Fraunhofer lines of the spec- 

 trum should show similar characteristics and the diffraction 

 pattern should differ from those observed. Thus the conclu- 

 sion is apparently justified that distinct and independent points 

 of the narrow slit whose distance apart on its length is not 



