456 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Microscope there should be enough room for the hands to move com- 

 fortably and to be able to draw. 



The action of the dark box is that it strengthens the retina 

 wonderfully in the perception of the finest details. This takes place 

 in two ways. First, in the ordinary mode of observing with the 



Fig. 65. 



Microscope, the eye of the observer is so much disturbed by the light 

 from the illuminated eye-piece setting, and the surrounding objects, 

 that many microscopists are accustomed to shade the eye with the 

 hollowed hand as a remedy in delicate observation. This is obviated 

 in the most perfect manner by the dark box. In the next place, it is 

 by no means a matter of indifference whether strong or weak light- 

 impressions are simultaneously received by the other open eye, which 

 is at rest. Every more intense light-impression prejudices the sight 

 of the other eye more than is commonly supposed. Into the dark box, 

 however, only a faint illumination can enter from the light of the 

 room behind it, especially when the table is black. 



Feussner's Polarizing' Prism.* — Dr. K. Feussner gives a detailed 

 description of the polarizing prism lately devised by him, which 

 presents several points of novelty, and for which certain advantages 

 are claimed. The paper also contains an account, although not an 

 exhaustive one, of the various polarizing prisms which have from 

 time to time been constructed by means of different combinations of 

 Iceland spar. 



I. Older Forms of Polarizing Prisms. — In comparing the various 

 forms of polarizing prisms, the main points which need attention 

 are : — the angular extent of the field of view ; the direction of the 



* Zeitschr. f. Instrumentenk., iv. (1884) pp. 42-50 (8 figs.). See P. E. Slee- 

 man in ' Nature,' xxix. (1884) pp. 514-7 (8 figs.). 



