ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 



403 



circular patterns of stop were compared experimentally by using a con- 

 denser fitted with two stop carriers, one behind the other, so that either 

 stop could be used separately, or both together. The twin stop used by 

 itself gave a good dark ground. The circular stop was purposely chosen 

 too small to give a good dark ground ; there was light at the sides of 

 the field. Nevertheless when the circular stop was turned in above the 

 twin stop whilst the object was under observation, there was a marked 

 drop in the brightness of the image. This loss of fight was due almost 

 entirely to the circular stop, not to the clear white glass on which it was 

 mounted, since it was found that the interposition of such a piece of 

 glass, even when rather dirty, made very little difference to the light. 

 Evidently, therefore, the circular stop, though too small in one direction, 

 was too large in the other, and kept out some rays which might safely 

 have been admitted. Of course, if the circular stop had been large 

 enough to darken the background when used by itself, the loss of light 

 would have been still more noticeable. 



Changer for Use with Substage Condensers.* — S. C. Akehurst has 

 frequently felt the need of a method for quick change of substage con- 



rv 



{ ^\ 



Pig. 40. 



densers. He found the revolving nose-piece to carry three condensers 

 did not work satisfactorily, and therefore adapted the principle employed 



Journ. Quekett Micr. Club, xii. (1914) pp. 277-8 (2 figs.). 



