91 



Accordingly, the light from the vacuum tube at a passes through 

 an ordinary spectroscope be, and the light from only one of the 

 lines in the spectrum thus formed is allowed to pass through the 

 slit d into the interferometer. 



As explained above, the light divides at the plate e, part going 

 to the mirror /, which is movable, and part passing through, to 

 the mirror g. The first ray returns on the path feh. The sec- 

 ond returns to e, is reflected, and passes into the telescope h. 



The resolving power of the interferometer is measured by the 

 number of light-waves in the difference of path of the two in- 

 terfering pencils, and as this is unlimited, the interferometer fur- 

 nishes the most powerful means for investigating the structure 

 of spectral lines or groups. Its use is, however, somewhat 

 handicapped by the fact that the examination of a single group 

 of lines may require a considerable number of observations 

 which take some time and during which it may be difficult to 

 prevent changes in the light source. Nevertheless it was found 

 possible by its means to investigate the wonderful discovery of 

 Zeeman — of the effect of a magnetic field on the character of 

 the radiation from a source subjected to its influence — and the 

 results thus obtained have been confirmed by methods subse- 

 quently devised. 



One of these is the application of the echelon. This is in 

 effect a diffraction grating in which high resolving power is 

 obtained by using a very high order of spectrum into which 

 practically all the light is concentrated. The number of elements 

 may be quite moderate — since the resolving power is the product 

 of the two. The order of the spectrum is the number of wave 

 lengths in the retardation at each step. This retardation (which 

 ?'nust be very accurately constant) is secured by allowing the 

 incident light to fall upon a pile of glass plates optically plane 



