8 



THE INTERFEROMETRY OF 



but appearing 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 superimposed spectra. 



It is possible, however, as will be shown in 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 them. 



The methods used in this paper consist chiefly in bringing the two first- 

 order spectra of a grating, or the second-order spectra or their equivalents, 

 to interfere. In this respect they contain an additional method of inter- 

 ferometry which may be useful, if for any reason it is necessary that the two 

 component beams are not to retrace their paths. 



2. Coincident spectra with one reversed on a given Fraunhofer line. In 



figure i, L is a narrow vertical sheet (subsequently broadened by the dif- 

 fraction 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 gN originate. M and N are opaque mirrors mounted 

 adjustably on a firm rail, RR, each of them with three adjustment screws 

 relative to horizontal and vertical axes. M is provided with a slide micrometer 

 (not shown). From M and N 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 be far preferable, 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 and N. More- 

 over, as the spectra are mirror images of each other, as suggested in figure i , 

 any spectrum lines (as, for instance, the D} 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. 



