REVERSED AND NON-REVERSED SPECTRA. 



29 



to their faces. P' is on a micrometer with its screw parallel to bb f , so that 

 this prism is shifted right and left. The range of displacement was found at 



M, about 0.04 cm.; :r = 2X0.04X0.939 = 0.076 cm. 

 P', about 7 = 0.07 cm.; 27 = 0.140 cm. 



where # = 200)3(90 0)/2 and 2y are the corresponding path-differences 

 between the inception and evanescence of fringes. With a very fine slit, 

 2y was possibly smaller (see fig. 17). 



The question at issue is thus, in the first place, how the value of zy compares 

 with x; for in the former case the angle 5 is effectively zero. In other words, 

 when M is displaced from M to M', over a distance e, the pencil b, figure 17, 

 changes to fej, and is soon lost at the edge of P', whereas, when P is displaced 

 in the direction bb r , over a distance y, the rays b and b f do not change their 

 point of impact at the prismatic mirror P'. If PP represents the principal 

 plane of the objective of the telescope and F its principal focus, there should 

 be no accessory effect for the case y as compared with the case x. 



Results bearing on this subject are given in table 9, in which the displace- 

 ment e, observed at M and at N, as well as the displacement y at P', are 

 recorded when a plate of glass of thickness E is inserted normally to the 

 rays b, b'. The corresponding air-path difference computed from E, n, B, 

 and X should be z, nearly. This is about the value (27) observed, remembering 

 that to set the micrometer, fringes of a particular pattern must be selected. 

 The rotation of fringes being but 90 or less, there are no fiducial horizontal 

 lines. 



TABLE 9. Reversed spectra. Refracting prism. = 49 45. 5=4.6X10-" (assumed). 



x = 2ecos (go-d)/2;z = E ( M -i+25/X 2 ). 



