56 THE INTERFEROMETRY OF 



(D = 162 X iQ- 6 ) . As a result of the two diffractions and the half-silver reflec- 

 tion, the spectra were too dull to make it worth while to look for fringes, at 

 length. None was discernible after some searching, to my regret, for the 

 method itself has many points of interest and with gratings of low dispersion 

 it succeeds. 



Later I stripped a celluloid film from the ruled grating (D = 352 X io~*) and 

 mounted it by simple stretching. Using the original grating at G and the film 

 at G', the fringes were found with some patience. The spectra were fairly 

 bright (arc lamp) and the fringes reasonably strong; but they admitted of a 

 displacement of only 1.8 mm., in spite of the vanishing angle d = o. Possibly 

 this small displacement is due to the imperfect film grating. 



In further experiments I half-silvered plates of glass to different density. 

 In this work I obtained adequately bright spectra and practically perfect 

 fringes, but the range of displacement (5 = o) could not be increased above 

 1.8 mm. Within this interval the fringes seem to change form but little, 

 thinning only being evident. Then they become dull and vanish. 



The method was temporarily modified, as shown in figure 36, where G is a 

 ruled grating (> = 352Xicr 6 cm.), G' a film grating (.0=176X10-* cm.), and 

 m an opaque mirror. This naturally introduces an angle 8=6' 6, which is 

 negative when 6' > 6, or the grating G shows greater dispersion. The mirror 

 is limited in breadth, so that the rays a, a' have free access to M and N. The 

 fringes were found after some trouble, for the transmitting grating G' also 

 acts as a reflecting grating if the rays b, b f fall upon it, and it is not always 

 easy to separate these two cases, each of which will give fringes on proper 

 adjustment. The spectra are very bright and the range was about 2 mm. 



The same method was now carried out with prisms, as shown in figure 37, 

 where L is the incident pencil from a collimator, P and P' right-angled prisms, 

 / the half-silvered plate, N and M opaque mirrors on micrometers, T the 

 telescope. The spectra seen at T have each been four times refracted and 

 twice reflected, at M and H. They are very bright, so that a very fine slit 

 (here specially desirable) is available. The sodium lines are not separated. 

 On bringing the spectra to overlap at their corresponding edges, the fringes 

 were found. They are peculiar, inasmuch as they show the phenomenon of 

 figure 32, but with the faint fringes curved and more prominent than hereto- 

 fore. In other words, the faint phenomenon shifts across the field from side 

 to side, but is enormously accentuated at the transverse strip of the linear 

 phenomenon. Narrowing the incident pencil broadens and blackens the 

 fringes. They may be obtained in the gap between two spectra just separated. 

 The range of displacement within which fringes are seen was, however, very 

 small, not exceeding 0.2 mm. This is a characteristic of these fringes and in 

 keeping with the low dispersion. 



It is interesting to note that the systems figures 35, 36, 37 constitute an 

 element of a direct-vision spectroscope. It has been shown that it can be 

 made quite powerful. 



The same method, figure 37, was now used with two 60 prisms of highly 



