652 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



then H will be invisible, and vice-versa. The remedy is to bring both 

 A and H to the same optical distance from the eye, and this may be clone 

 by inserting a convergent lens of 2*33 dioptries (i.e., 3*33 - l'O) 

 between the eye and the paper. The author discusses many of the 

 possible cases, and gives tables of the lenses required under various cir- 

 cumstances. 



New Nicol for Projection Purposes.* — W. von Ignatowsky points 

 out that for the projection of crystalloptic examination it is usually 

 necessary to have a parallel beam of polarized light, and that the diameter 

 of this beam must be as great as possible. The polarization of the beam 

 is effected in two ways, either by bringing the polarizer directly into the 

 beam, or into the place where the image of the light-source is formed. 

 In the latter case, it is necessary to apply a lens behind the polarizer (in 

 the sense of the light-course) in order to parallelize the beam. This is, 

 indeed, the method usually adopted, because it requires a smaller nicol. 

 In the former case, the free aperture of the nicol must be very great, 



Fig. 91. 



and the method is, owing to the high price of calcite, expensive. But 

 both methods have the following disadvantage. As 50 p.c. of the in- 

 cident radiation remains behind at the nicol, the nicol itself becomes so 

 heated within a very short time that the cement layer is injured, and the 

 nicol is useless for further investigation. 



The author describes a method by which the above inconvenience is 

 avoided in a very simple way ; it even allows an hour's projection with 

 arc lamp and a current up to 30 amperes without the least injury to the 

 nicol. The cause of the heatiDg lies, doubtless, in the absorptive pro- 

 perties of the black layer which reflects the ordinary ray. It therefore 

 becomes necessary to avoid this cause. Fig. 91, representing a nicol 

 with perpendicular end-planes, shows how the author gets over the diffi- 

 culty. The extraordinary ray o X goes right through the nicol, and the 

 ordinary ray o a is reflected at the cement layer D C. The plane A C of 

 the nicol is polished. But as the angle a is greater than the angle of 

 total reflexion, a glass prism A B C is cemented on, thereby permitting 

 the free exit of the ordinary ray oad out of the nicol. In the applica- 



* Zeitschr. f. Instrumentenk.,xxx. (1910) pp. 217-18(1 fig.). 



