720 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



Pringsheim's G-as Chambers.* — In order to make experiments 

 witli different gases, Professor N. Pringsheim had gas chambers con- 

 structed by Schmidt and Hansch, for use with his Photo-chemical 

 Microscope,! which differ from those hitherto used, and which combine 

 great firmness and durability with easy management. As those of 

 glass are very difficult to fix, besides having other disadvantages, the 

 new ones (Figs. 171 and 172) are of metal, and very firm and secure. 

 The base is of strong glass (or metal with an aperture closed with 

 glass), the sides and cover d of metal. The latter has a circular 



Fig. 171. 



Fig. 172. 



^ 



^( 



t 



aperture in the centre, beneath which a glass cover is cemented for 

 the reception of the hanging drops in which the object is placed. 

 It can be firmly pressed down by the arm 7c (movable at ^) and the 

 screw 8. By a mixture of wax and vaseline at the joints and 

 tightening the screws, the chambers can be made completely air- 

 tight, and will even bear a considerable 

 pressure of gas. This is conducted 

 through the tubes g. The base of the 

 hiamber is kept covered with a thin 

 ;ratum of water. As the temperature 

 f the drop, particularly in white light, 

 lay become higher than the object 

 in endure without injury, it may be 

 3oled by filling the chamber with ice, 

 and by placing on it, instead of d 

 (Fig. 171), the cover r (Fig. 172), which can then also be covered 

 with ice. In the latter case, a quick conductor of heat from the drop 

 to the ice can be obtained by means of the platinum cross p. 



Test for the Hand-Lens. — Mr. J. Deby points out that "while 

 many tests exist for high- and medium-power objectives, none are on 

 record for that most useful instrument to the naturalist, the hand- 

 lens." The best test he considers to be the elytron of Gyrinus 

 marinus, a not very rare water-beetle. The lens must not only show 

 the longitudinal rows of large dots, but also the fine intermediate 

 punctations. None but a first-rate lens will show them. The male 

 has finer punctations than the female, and is more difficult of 

 resolution. 



* Zeitschr. f. Instrumentenk., i. (1881) pp. 332-3 (3 figs.), 

 t See this Journal, ii. (1882) p. 395. 



