198 



THEORY OF MICROSCOPIC OBSERVATIOX. 



reflected at R and Q, and refracted for the second time in P, the 



triangles C P Q, C Q R, and CRT 

 are similar, and their angles adjacent 

 to the chords P Q, Q R, and R T 

 are = a. Consequently the three 

 angles P C Q, Q C R, and R C T at 

 the centre are together = 3 (180 

 2 a'). On the other hand, the triangle 

 PCD gives the relation 



= FOP - 



a, 



or, since 



FC P = 



360 -POT 



FIG. 108. 



360 - 3 (180 - 2 a) 

 P = ~ a 



= 3 a - a - 90. 

 For the case when the incident ray 

 emerges without deviation, therefore 

 p = 90, the condition accordingly is 

 3 a a = 180. This condition is satisfied if a = 46 28', from 

 which the distance of the point D from the centre is calculated 

 at '725 . r. The following table, in which the same quantities 

 have been inserted as in the previous one, shows that parallel rays 

 whose angle of incidence is slightly greater or less than 46 28', 

 form, as in the previous case, a virtual focus whose smallest 

 diameter lies somewhat above the plane of adjustment and very 

 near to the line of the ray emerging without deviation. 



Angle of Incidence 



a. 



Distance of the points of inter. 



TkairfaH^n fmm the Pprrwm Distance of the points of inter- 

 Deviation from the Perpen- , 8ectlon , n fche f ane of ad 



dlCUlar <J>. mnt. frnm fh Pi,fr 



ment from 



The outer bright ring is consequently at a distance of '725 . r 

 from the centre; and from the inner one "0366 . r, or approxi- 



