314 



POLARISATION. 



should not give the preference to any other position of the 

 Nicol. 



The position of the analysing Nicol above the eye-piece necessi- 

 tates, moreover, in most Microscopes that it shall be of considerable 

 size, so that the field of view may not be too contracted. Further, 

 the position of the eye here enters into consideration, and the 

 convergence of the cones of light connected with it. We will 

 examine more closely the influence of these two factors. 



Confining ourselves to a definite case. In Beneche's No. 2 

 eye-piece, the diameter of the optically effective portion of the 

 eye-lens is about 3*5 mm. The peripheral 

 cones of light which correspond to the marginal 

 points of the field of view are inclined at an 

 angle of about 16 to the optic axis, and the 

 eye-point is situated about 6 mm. above the 

 eye-piece. If we demonstrate this relation by 

 an accurate construction (Fig. 174, where the 

 pencils of light are delineated by simple lines), 

 and suppose a Nicol applied with its mounting 

 B, inasmuch as the displacement of the rays 

 parallel with themselves may be disregarded, 

 the following conclusions immediately obtain: 

 (1.) The peripheral pencils of rays must be 

 cut off by the diaphragm at d up to an inclina- 

 tion of about 12 (the limiting angle of the total reflexion for the 

 ordinary rays). The field of view is thus reduced. 



(2.) As the diaphragm is situated considerably above the 

 level of the eye-point, the eye surveys at one time only a small 

 portion of the field of view, and must be moved over the 

 diaphragm in order to perceive all parts of the field. This incon- 

 venience is the greater, the longer the mounting of the Nicol in 

 relation to the distance of the eye-point. We might, however, 

 construct the eye-piece so that the eye-point would lie in the 

 plane of the diaphragm, which arrangement would also involve a 

 less inclination of the rays to the axis. The eye would then 

 survey at one time the whole field of view of the Microscope from 

 d, and it would be unnecessary to cut off the peripheral rays. 

 The diameter of the Nicol would of course have to correspond to 

 the effective part of the eye-lens. 



If we are satisfied with a small field of view, such as is given by 



FIG. 174. 



