C. Barus — Interferences of Crossed Spectra. 487 



line and be again inappreciable. Experimentally, I was not 

 at first able to find any interferences for this case in the earlier 

 manner shown below. But this may have been due to 

 inadequacies in the experimental means employed : for the 

 dispersion was insufficient and the reflecting edge of the paired 

 mirrorstoo rough. Improving the apparatus, I eventually found 

 the phenomenon, appearing however as a single line, vividly 

 colored above the brightness of the spectrum ; or again more 

 jet black than the Fraunhofer lines and located in the position 

 of the coincident wave lengths of the two supei-imposed 

 spectra. 



It is possible, however, as will be shown below, §4, to obtain 

 two spectra in such a way that if their longitudinal axes 

 coincide the Fraunhofer lines intersect at a small angle, and 

 vice versa. In such a case, for coincident Fraunhofer lines, 

 interference occurs in a band around these lines and is absent 

 in the rest of the spectrum ; whereas, if the longitudinal axes 

 are coincident, the interferences are arranged with reference 

 to these axes. These results seem to bear on the question, 

 but it is difficult to clearly resolve it. 



The methods used in this paper consist chiefly in bringing 

 the two first order spectra, or the second order spectra or their 

 equivalents, to interfere. In this respect they contain an addi- 

 tional method of interferometry wdrich may be useful, if for any 

 reason it is necessary that the two component beams are not 

 to retrace their paths. 



2. Case of coincident spectra with one reversed on a given 

 Fraunhofer line. — In figure 1, L is a narrow vertical sheet 

 (to be broadened by the diffraction of the slit) of white sunlight 

 or arc light from a collimator, G the transparent grating ruled 

 on the side g from which the first or second order of spectra 

 gM and gJV originate. M and JV are opaque mirrors, mounted 

 adjustably on a firm rail, RR, each of them with three adjust- 

 ment screws relative to horizontal and vertical axes. M is 

 provided with a slide micrometer (not shown). From M and 

 i^the beams pass to the smaller paired mirrors, m and n, 

 which should meet in a fine vertical line at a very obtuse 

 angle. A silvered biprism would here have been far prefer- 

 able, but none having the required angle was available. 

 From n, m, the beams pass into the telescope T. As the 

 spectra are each divergent after issuing from g, they can be 

 made to overlap on leaving n, m, by aid of the adjustment 

 screws on M JV. Moreover as the spectra are mirror images 

 of each other, as suggested in figure 1, any spectrum lines 

 (as for instance the I)) may be put in coincidence on using 

 one of the adjustment screws specified. It is necessary that 

 the telescope T be sufficiently near M in order that the 

 micrometer may be manipulated. 



