134 



THE INTERFEROMETRY OF 



TABLE 36. 



93 



4. 



Jt 



71- 



These data contain the new result that the achromatic fringes decrease in 

 size with great rapidity in both directions of the increments from the cen- 

 tral position (a = o). Two sets were usually present. The rate of growth as 

 well as the ratio AW/A a is not the same on both sides of the position of maxi- 

 mum size (a = o). The mean coefficient may be found graphically as 



AN 



- = 0.56 cm. /degree or 32 cm./radian 

 Aa 



which is larger than the preceding estimate for spectrum fringes. 



This experiment bears directly on the nature of the achromatic fringes. 

 It shows, moreover, that the compensators C and C' in figure 88 are not 

 essential and that ellipses may be centered 

 by rotating the mirror m on a horizontal 

 and vertical axis. It is, in fact, possible 

 to increase the achromatic fringes indefi- 

 nitely in size in this way; but with ordinary 

 glass they eventually become sinuous and 

 no longer useful. When the slit is fine 

 and the ocular out of focus, well-marked ^ 



hyperbolic patterns may be recognized; but these become straight on return- 

 ing to the wide slit in focus. 



69. Measurement of small angles without auxiliary mirror. This method 

 makes use of the original apparatus, figure 73, but the two mirrors M and M' 

 or N and N', figure 93, are rotated together as a rigid system around a ver- 

 tical axis, at A, for instance. In view of the absence of auxiliary reflection, 

 the method will be but half as sensitive as the preceding one, so that the 

 equation 



c/l 



= cos 



is sufficient to express the results. But on the other hand, if spectrum fringes 

 are to be observed, there is greater abundance of light, since the half-silver 

 film is penetrated but once by each component, ac or bd. When the achro- 

 matic fringes are used the light is always superabundant and must be reduced 

 in intensity. To try this method the mirrors N and N' were mounted on a 

 good divided circle so as to rotate together on a rigid arm over a small angle a. 

 The achromatic fringes displaced in this way were restored by advancing 



