ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



711 



small. The best is 1 to 3 /*. If the distance is much greater there is 

 much disturbance from the indistinctness of the extra-focal parts of the 



Fig. 117. 



image ; ifHhe distance is less there, is a very disturbing adsorption 

 effect of the glass planes on the ultra-microscopical particles. 



Sjedentopf, H. — TJeberdie physikalischen'Principien der Sichtbarmachung ultra- 

 mikroskopischer Teilchen. 



[This is substantially the same as Dr. SiedentopPs lecture before the Society, 

 June 17, 1903. and printed in the Journal, October 1903.] 



Berliner Klinischer Wochensch., 1904, No. 32 ; also 

 reprinted as an extract, 7 pp. 



(4) Photomicrography. 



Grain in Photographic Plates.* — R. J. Wallace gives an account 

 of the circumstances which control the size of the silver particles in a 

 developed gelatino-bromide plate. Generally speaking, these particles 



* Astrophysical Journal, Sep. 1904. See Nature, lxx. (1904) p. 571 (1 fig.). 



3 C 2 



