CHAPTER VII. 



PRISMATIC LONG-DISTANCE METHODS EN REVERSED AND NON-REVERSED 



SPECTRUM tNTERFEROMETRY. 



51. Purpose. It is preliminarily the object of the present paper to examine 

 a variety of new methods for the production of interferences with spectra, 

 with a view to the selection of as simple a design as possible for practical pur- 

 poses. Some interesting differences appear in the results, so that the sim- 

 plicity of construction does not necessarily recommend the apparatus for use. 



In the second place, the endeavor will be made to assemble appurtenances 

 in such a way that the extremely mobile phenomena may be under control, 

 even in a moderately agitated laboratory. In case of the early interferometer 

 experiments, the interferences disappeared on merely touching the apparatus, 

 and are rarely or never at rest; whereas it is, of course, necessary that they 

 should remain visible while the micrometer is being moved. These experi- 

 ments are now nearly completed, but will preferably be described in a succeed- 

 ing report. 



52. Methods and apparatus. Some prismatic methods were tested in the 

 earlier volume, but not developed; for the plan of using a transmitting grating 

 twice, or two gratings in succession, seemed to contain greater promise. The 

 prism method is, however, more sim- 

 ple than any of the others and there- 

 fore deserving of special study. 



In figure 72 the large right-angled 

 prism P, with its faces silvered, re- 

 ceives the pencil of parallel white 

 rays, L, on its orthogonal faces and 

 reflects them to the plane opaque 

 mirrors n and m. From here the rays 

 are further reflected, either nearly in 

 parallel, as in the figure, or crossed, 

 as at c, c', to the remote opaque 

 mirrors N and M, which in turn re- 

 flect them to the plane or concave 

 grating G. If the rays converge at the 

 appropriate angle of diffraction, 6, a 

 selected color will be diffracted in the direction of the normal to G in each 

 case. If the two paths are nearly equal, these rays will therefore interfere 

 in the axis GT and the results may be observed by a telescope or a lens at T. 

 In my apparatus the distances mM and nN were of the order of 2 meters. 

 In consequence of the three successive reflections, it is somewhat difficult to 

 102 



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