ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



369 



temperature, the alkali vapour re-distils out of the salt whereby the 

 crystal is decolorised. The temperature is also limited by the melting 

 point of the salt (about 800°), and by the bending temperature of the 

 combustion tube (about 700°). After the heating the preparation tube 

 is taken out. of the furnace and allowed to cool in a vertical position in 

 order to avoid external smearing of the coloured crystals with alkali 

 vapour. The excess of alkali settles down in the trough-shaped 

 bottom of the tube. After cooling, the tube is carefully 

 snapped off, without breaking the crystals, which are 

 then rapidly rinsed with water and dilute hydrochloric 

 acid and then dried. For microscopic examination, the 

 preparation is then ready ; but for ultramicroscopic in- 

 vestigation they must, on two adjacent surfaces, be 

 polished as carefully as possible free from scratches and 

 the polished surfaces made permanent by means of cover- 

 glasses cemented on. 



The ultramicroscopical examination is performed by 

 the usual apparatus, and Siedentopf concludes that rock 

 salt coloration is essentially due to separation of ultra- 

 microscopic metallic sodium crystalline particles, mostly 

 needle- or flat-shaped, and partly pleochroitic, which 

 are irregularly distributed within the ultramicroscopic 

 cleavage planes and which vary from cubic centimetre to 

 cubic centimetre of the crystal. 



He gives his reasons in full for his conclusions, re- 

 jects the hypothesis of sub-chloride formations, and 

 refers to the phenomenon of electric resonance previously 

 noted by W. Wood.* 



The author adds a coloured plate comparing the results of artificial 

 and natural colorisation as furnished by his ultramicroscopical analysis. 



Some Simple Questions on the Images of Microscopes and Tele- 

 scopes.! — At a recent meeting of the Physical Society, W. B. Croft 

 stated that : " It may have been noticed that when a Microscope is 

 focused visually, an image is formed on the focusing-glass of a camera, 

 into which the microscopic eye-piece is inserted after removing the 

 camera-lens. This image remains more or less in focus at variable 

 positions of the camera-screen. Although it is not always perhaps true, 

 yet it is surprising how often the pencil emerging from a Microscope 

 eye-piece behaves like a single concentrated line of light. A mounting 

 of the proboscis of a fly was focused visually, then a camera was put on 

 the end of the Microscope, and the screen was racked out to six positions 

 over a range of about 6 in. At these positions six photographs were 

 taken, of which the two' smallest pictures were inferior to the others. 

 It was claimed that this defect was due to imperfect exposure and 

 uneven illumination. If the visual focusing had made pencils slightly 

 diverging instead of parallel, the larger pictures should have been the 

 more imperfect. These pictures were obtained with a 1-in. objective. 



* Phil Mag. (6) iii. (1902) p. 396. 



t Proc. Phys. Soc. London, April 27, 1906. 



June 20th, 1906 b 



Fig. 47. 



