416 Prof. Carl Barus on Interferometry 



some differences in the glass, has produced no discernible 

 effect. 



An interesting case is the film grating between two 

 equally thick plates of glass. With this, in addition to the 

 elliptical interferences above described, a new pattern of 

 vertical interferences identical with those discussed in a 

 preceding paper * were obtained. These are linear, per- 

 sistently vertical fringes, extending throughout the spectrum 

 and within the field of view, nearly equidistant and of all 

 colours. Their distances apart, however, may now be passed 

 through infinity when the virtual air-space passes through 

 zero ; and for micrometer displacements of mirror in a given 

 direction, the motion of fringes is in opposite directions on 

 different sides of the null position of the mirror. I have not 

 been able, however, to make them as strong and sharp as 

 they were obtained in the paper specified. 



3. The Three Principal Adjustments for Interference. — To 

 compute the extreme adjustments of the grating when the 

 mirror N is moved, fig. 5 (below) maybe consulted. Let y 

 be the air-path on the glass side, whereas y a is the air-path 

 on the other, e the thickness of the grating, and fju its index 

 of refraction for a given colour. Then for the simplest case 

 of interferences, in the first position N of the mirror, if I is 

 the normal angle of incidence and K the normal angle of 

 refraction for a given colour, 



y ff +ep./ooa'R*=i/ a 



for equal paths. Similarly, in the second position of the 

 mirror, 



Hence, if the displacement at the mirror be N, as the figure 

 shows, 



Va~ y^=2*tanB,sinI, 



N = 2<?yu,cosR. 



The measured value of this quantity was about 1*88 cm. 

 The computed value would be 2 x '68 X 1*53 x •71 = 1-84: cm. 

 The difference is due to the wedge-shaped glass which 

 requires a re-adjustment of the grating for the two positions. 

 The corresponding extreme adjustments, when the grating 

 gg instead of the mirror N is moved over a distance s, are in 

 like manner found to be 



z = t — ■— tt — e tan I tan B. 



cos 1 cos. ft 



* Phil. Mag. I c. 



