516 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



rays reflected at L are thrown aside and do not reach the second prism. 

 This adjustment is described as easy and rapid. 



Fig. 78 shows the complete apparatus, as built up by Messrs. Leitz, 

 on Le Chatelier's principle. The object is simply laid on the stage T, 

 and adjustment is performed by stage-movement, the other parts of the 

 apparatus being unmoved. By means of the second prism P (fig. 77) 

 the rays required for vision are switched off to the left, and the 

 actual observation is through and a corresponding prism. If this 

 last prism be taken out, the photographic apparatus comes, into action. 

 A small lamp of constant current strength (4 amperes) with hand- 

 regulation serves as the li°'ht-source. 





Fig. 78. 



Microscopical Observations with Dark-ground Illumination.* — In 

 this article H. Siedentopf discusses the following points connected with 

 dark-ground work:— (1) Brightness of condenser; (2) criteria for 

 under- and over-correction ; (3) cover-glass correction due to tube- 

 movement ; (4) observations with immersion-systems ; (5) advantages 

 of dark-ground illumination with coloured preparations ; (6) variations 



mages. 



In dealing witli the first head, the author describes a method of 

 testing the brightness of a condenser by directly observing the visi- 

 bility of the issuing rays. A black glass plate G- (fig. 79) and a small 

 prism A are cemented on to the underside of a cover-glass D, leaving 

 between them a hollow space 15 mm. long, 3 mm. high, and 2-3 mm. 

 broad. This space is to receive a fluorescing immersion fluid, cedar- 



* Zeitschr. wiss. Mikrosk., xxv. (1909) pp. 273-S2 (3 figs, and a plate of 9 micro- 

 photographs). 



