ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



803 



In order to introduce a needle into the capillary tube upon the slide, 

 a special protector is necessary. This is shown in fig. 143 where it 

 appears as a black tube fastened to the slide by shellac G. The difference 

 between the parts sliding on one another must not amount to more than 

 10 IX. The lever L, figs. 142 and 143, is fixed to the capillary tube with 

 a minute drop of marine glue. The other extremity lies upon the screw 

 Y fixed to the standard of the Microscope at A, and between the milled 

 head E and the spring E, made of brass wire. The perforation of an 

 ovum is effected by just flicking the spring after having turned the screw 

 back to the required degree. 



There are numerous minute details given by the author as to points 

 of manipulation, but for these the original must be consulted. 



Measuring Corrosion Surfaces in Iron Pyrites.* — Herr F. Beeke, 

 while examining iron pyrites, came to the conclusion that the primary 

 corrosion surfaces were those of greatest resistance, and in order to prove 

 this measured the difference between several parallel surfaces on the 

 same crystal. For this purpose 



a screw micrometer by Zeiss Fig. 144. 



was used in conjunction with 

 an apparatus (shown half its 

 natural size, fig. 144) for 

 measuring the thickness of the 

 crystal under the Microscope. 

 To the metal plate A inter- 

 rupted at O, the upright piece B is attached, and to this a piece of 

 plate glass E is fixed. Upon A are also fixed two more uprights C D, 

 through which the screws S and F work. The screw S is rounded off 

 at one end, pointed at the other, and bears a milled head. The screw 

 F is pointed at one extremity, and at its other terminates in a milled 

 head. This screw during the experiments is fixed. The crystal K is 

 placed between the glass plate and the screw S, which is made to fix it 

 closely both before and after corrosion. Then the difference in distance 

 between the points S and F shows the amount of substance lost. 



Rowland's Reversible Compressorium. — This device of Mr. W. 

 Kowland (fig. 146) consists of two thin German silver plates each with a 



Fig. 145. 



ring having a piece of cover-glass cemented to it. The lower plate is 

 attached to a rod turning in a socket, while the upper pivots on a milled 



* Tschermak's Mineral, u. Petrogr. Mittheil.. viii. (1887) p. 318. Cf. Zeitschr. 

 f. Wiss. Mikr., iv. (1887) pp. 411-2 (1 fig.). 



