20 THE INTERFEROMETRY OF 



to carry this pencil in an opaque tube reaching from the objective of the 

 collimator, as far as the grating. 



With first-order spectra this method always succeeded satisfactorily, and 

 in case of a ruled grating the phenomenon is exhibited brilliantly, if the paths 

 GM and GN are optically nearly equal. After some experience it is fairly 

 easy to find it. I have not, however, been able to obtain it with a film grating, 

 even after using a variety of excellent samples. This is not remarkable, for 

 the film grating is hardly sufficiently plane to produce clear regular reflection, 

 and the corresponding paths GM and GN would not, therefore, be definite. 



Second-order spectra are too faint and can not be seen, unless the glare is 

 excluded in the manner stated. All modifications of the method seemed with- 

 out avail, until finally the light was led from the collimator objective C, 

 figure n, to the grating G, in a cylindrical tube, whereupon both the glare 

 from the objective and the rearward reflection from the grating were effec- 

 tively screened off. This tube must, of course, lie below the returning pencil, 

 i.e., it must not (in section) cover more than the lower half of the grating. 

 In this case the second-order spectra, though faint, were seen clearly; but 



the scintillating interferences could not be observed until the very weak 

 eyepiece, E, was used with the concave mirror m; or a weak telescope with 

 a plane mirror. It was then detected, but showed no essential difference from 

 the case of first-order spectra. The larger dispersion, in other words, was 

 unavailable. The phenomenon was seen most distinctly by drawing out the 

 eyepiece of the telescope, as the light is thereby concentrated, although the 

 Fraunhofer lines vanish. Second-order spectra are therefore not necessarily 

 advantageous. The phenomenon is very hard to find, and the experiments 

 were persisted in only to obtain the result under different conditions. 



The tube-like light conductor referred to above is, of course, advantageous 

 in case of first-order spectra. If the concave mirror is used, the phenomenon 

 may even be seen brilliantly with the naked eye. 



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, figure 12, is transmitted by the grating (ruled, 

 half-silvered face, g toward the mirrors M and AT) and the two diffracted 

 beams then returned by the opaque mirrors M and N, to be in turn diffracted 

 by reflection into the telescope T. In fact, this method succeeds with the 

 unsilvered grating; for the rays diffracted, by reflection, from the ruled face 

 (toward the telescope), but not very well. The reflection from the rear face 



