ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 605 



preceding, as far as the displacement of the colours is concerned. If 

 the light is polarised in the plane of incidence, the colours observed are 

 very brilliant, and purer than with white light. If the light is polarised 

 in a plane perpendicular to the plane of incidence, the colours become 

 more and more dull. For an angle of 45° in the gelatin they disappear 

 entirely, and are replaced by a uniform yellowish tint due to the 

 reflecting silver produced by the development. 



3. Without the Mercury Mirror. — The results are, in all cases, 

 analogous to the foregoing. For i x = 45° there is total reflection at the 

 gelatin-air surface ; with light polarised in the plane of incidence the 

 photograph gives beautiful colours in the region affected by the blue ; 

 in the other case there are no colours, but a reflecting surface of 

 yellowish silver. 



The author has noticed that under the same conditions of exposure 

 and of light the colours seen at any point of a plate are not the same as 

 with a reflecting mercury surface. 



Photography of the Absorption Rays of the Colouring Matters 

 of Blood.* — L. Lewin, A. Miethe, and E. Sterger have overcome the 

 technical and experimental difficulties connected with this investigation. 

 They have examined the blood of human beings, horses, pigs, rabbits, 

 frogs, and of earthworms ; also pure oxy-hasmoglobin and its products of 

 transformation ; also colouring matters derived from the blood. The 

 apparatus used comprised a spectrograph with a Thorpe grating giving a 

 a spectrum of 9 cm. long between wave-lengths 800 and 300. A quartz 

 spectrograph was used for the ultra-violet to see if the absorption rays 

 existed beyond 300. The luminous source was ignited threads of 

 magnesium, which not only gave a very clear regular light, especially in 

 the violet and ultra-violet parts of the spectrum, but also some very 

 sharp lines serving to locate the absorption bands. In some experi- 

 ments the oxy-hydrogen-zirconium light was used, and in the less 

 refrangible parts a Nernst lamp. The liquids were contained in small 

 vessels with parallel sides or in test-tubes of known diameter. These 

 latter acted at the same time, if thought desirable, as convex lenses. 



Production of Stereo-Photomicrographs. — W. P. Dollman writes 

 that the following revision of his method f covers both high- and low- 

 power work. 



" The methods of producing stereo-micrograms are comparatively 

 simple. In an article by the late Mr. J. Traill Taylor in the 

 ' British Photographic Journal Almanac ' for 1894 are described 

 several methods of doing this work, one of the simplest of which is 

 that of obscuring by a semicircular screen half of the objective in use, 

 and using alternately the right and left hand (or upper and lower if 

 the plate is upright in the camera) halves respectively for the two 

 images required for the stereoscope. For this method may be sub- 

 stituted a plate with an eccentric circular aperture, as giving superior 

 definition to a semicircular opening. Placing a plate in front of the 



* Comptes Rendus, cxlii. (1906) pp. 1514-16. 

 t See this Journal, 1906, p. 257. 



