372 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



twelve apertures for illumination and observation. In the interior of the 

 drum is a mirror which is movable in all directions. A spring' register 



at'the back of the drum insures the correct position of each specimen as 

 it comes under observation. The Microscope is provided with a coarse 

 rack-and-pinion adjustment. 



Petri, R. T. — A. van Leeuwenhoek's Mikroskop. g 



Naturio. Woclienschr., sxii. p. 1-7. 



(3) Illuminating and other Apparatus. 



Polarising Prisms.* — B. Halle commences his treatment of this 

 subject by an interesting historical outline of the calcite prism. He 

 shows how the " epoch-making discovery of the Englishman Nicol " has 

 suggested other forms due to later observers, e.g. to Foucault. Praz- 

 mowsky, Glan, Glan-Thompson, Hartnack-Prazmowsky, Ahrens, Grosse, 

 Rochon, Senarmont, Wollaston, and Abbe. He describes and tabulates 

 the characteristics of each, and shows that the necessary waste of 

 material in their preparation increases rapidly in the more modern 

 forms. This is a serious matter on account of the growing rarity of the 

 raw material, the price of which has risen some twentyfold in the last 

 thirty years. The author has partially met this difficulty, for by the 

 help of a specially constructed saw | he has succeeded in reducing the 



* Deutsch Mechaniker-Zeitung (Jan. 1908) pp. 6-8 and 16-19 (3 figs.), 

 t Op. cit., 1896, p. 143. 



