ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



-;;; 



and white corpuscles are counted, the shutter of the ocular being at one- 

 half or one-quarter of the total field of vision. The count is made several 

 times through the same slit, and an average of corpuscles per field is 

 obtained. 



Now L : K : : I : r . ' . L = — 



r 



where L represents the unknown number of leucocytes, R the known 

 number of red corpuscles per cubic millimetre, and I and r represent the 

 average of leucocytes and red corpuccles respectively per field of vision. 



Pulfrich's Stereo-Komparator. — C. Pulfrich* has introduced an 

 improvement into the above instrument, designed a few years ago,f 

 which not only makes it better fitted for its original purpose, but also 

 adapts it for photometric and spectrographic measurements, as well as 

 for estimation of star magnitudes. In the earlier design the ray of 

 light impinged on an inclined and semi-opaque film of silver (fig. 29), 



Fig. 29. 



Fig. 30. 



and was thus partly reflected and partly transmitted. It was found 

 that the two images thus obtained, differed in intensity, thereby causing 

 difficulties in experiments of comparison. The author has now replaced 

 this silver film by a disk divided into ten sectors (fig. 80), five of which 

 are opaque and five are transparent. He gives full details of the appli- 

 cation of his design, which seems to have been highly successful. 



Ulbricht, R. — Zur Anwendung des Xugelphotometers und Zur Lichtschwer- 

 punkt-Bestimmung. Electrotech. Zeitschr., xxviii. (1907) p. 777 ; 



Op. cit, xxx. (1909) p. 322. 

 See also Zeit. f. Instrumentenk., xxix. (1909) pp. 353-6(3 figs.). 



(4) Photomicrography. 



Method of Preparing Stereo-photomicrographs.} — A. C. Banfield 

 describes a method by which he has met with considerable success. 



* Zeit. f. Instrumentenk., xxx. (1910) pp. 1-6 (8 figs.). 



t Tom. cit., xxiv. (1904) pp. 161-6. See also this Journal (1904) p. 578. 



X Journ. Quekett Micr. Club, 1909, pp. 459-64 (4 pis.). 



April 20th, 1910 R 



