THE POLAKISEK. 



307 



rays is, however, a varying factor which, according to Rudberg, 

 amounts at its minimum to 148635, and gradually increases with 

 the deviation to the left until the direction of the principal axis of 

 the crystal is reached. Since the latter in the given case forms an 

 angle of 63 44' with the perpendicular, each value for the different 

 deviations is determined by the curve of an ellipse whose major 

 axis coincides with the principal axis of the crystal, and is in the 

 ratio to the minor axis as 1-6585: 148635. Hence we obtain as 

 coefficient of refraction 1-5636 for a ray deviating in the crystal 

 15 to the left ; this value exactly suffices to cause total reflexion 

 at the layer of balsam which the ray meets at an angle of 82 IT 

 (the refractive index of the balsam assumed to be 1-549). If the 

 refraction at the lower face of the Nicol is also taken into account, 

 we get about 11 as the maximum inclination for the incident rays.' 

 In reality the inclination might be found from two to four degrees 

 less, as the refractive index of Canada balsam is given as somewhat 

 lower by Wollaston and Young. 



From the above investigations the 

 whole aperture of the incident cone of 

 light, in the direction from right to left, 

 therefore amounts to 12 58' -f 11 for 

 the one, and 16 44' + 7 for the other 

 extreme that is, in both cases, to nearly 

 24. This magnitude agrees tolerably 

 accurately with that determined experi- 

 mentally by Beneche with a Nicol prism. 

 Ill the direction vertical to the plane of 

 the paper the angles of inclination are 

 somewhat larger ; they are here deter- 

 mined solely by the dimensions of the 

 prism. 



This is not, however, equivalent to 

 saying that the whole of such a cone of 

 light reaches the field of view of the 

 Microscope. The path of the rays, 

 accurately drawn in Fig. 172, would in- 

 dicate that this is only possible under 



the most favouralle circumstances. The two lines m n and p q 

 represent two limiting rays, which pass through the prism in the 

 directions determined above, and suffer the deviation figured at 



FIG. 172. 



