92 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



If partially monochromatic light, like that of the arc lamp filtering through 

 ruby glass, is used, the graph obtained on successive exhaustions is sinuous, 

 indicating the alternate illumination of the disk and first ring; but it is not 

 sharp and clear, as in the case of strictly monochromatic light. 



PART II. DISPLACEMENT INTERFEROMETRY WITH FILM GRATING. 



52, Introductory, The transparent plate grating is expensive and rela- 

 tively unavailable, owing to the fact that the glass soon injures the diamond 

 edge of the ruling machine. It is therefore desirable to attempt to replace it 

 by the film grating, now so admirably made by Mr. Ives and others. Some 

 time ago I showed that this is quite possible, though the ellipses obtained were 

 not comparable in definition with those of the ruled glass plate. The following 

 paragraph is an attempt to improve the former method. 



The arms of the interferometer used were nearly 150 cm. long. Hence the 

 tests made are throughout severe. It is shown in the following paragraphs 

 that though any kind of film grating may be used, the particular type in which 

 the smooth side of the film is cemented to plate glass while the ruled side is 

 exposed is nearly as serviceable as the ruled plate grating. 



i Z 



f 



4 6 



r i 



c/lf 



f i 



FIG. 52. 



FIG. 53. 



53. Films between glass plates. This is the usual form of the grating in 

 the market, the unruled side of the film (15,000 lines to the inch) having been 

 attached with Canada balsam to one of the plates, while a thin film of air 

 separates the ruled side from the other plate. If this grating is used in 

 the interferometer there must be three reflections on one side of the grating 

 and three on the other, supposing that the ruled face of the film and the glass 

 face are practically continuous. 



Fig. 52 represents the case when the rays return upon themselves. The 

 white rays L from the collimator are separated into the groups i , 2 , 3 , reflected 

 respectively from the front, the rear, and the intermediate faces of the grating 

 GG' (gg being the film), toward the opaque mirror N, whence they return into 



