292 THE AMERICAN MONTHLY [Oct 



cases with an aperture of 1. N.A., which is the maximum 

 where the condenser is not used in immersion contact with 

 the under surface of the slide. The small size Abbe con- 

 denser fits into the fitting of the iris diaphragm which ac- 

 companies each instrument, The large Abbe condenser 

 has an iris diaphragm and swinging arm to carry a ground 

 glass or a blue glass (one each included). A green glass, 

 giving approximately monochromatic light, or a series of 

 patch stops may be supplied. 



The Beck achromatic condenser has an aperture of 1. 

 N.A., is a corrected achromatic system with an aplanatic 

 aperture of about -9 N.A., and is both for spherical and 

 chromatic aberration a more perfectly corrected conden- 

 ser than the Abbe form. It gives finer results, but re- 

 quires to be more carefully centred and adjusted. 



Wider angle condensers are supplied, but, of course, 

 can only make use of their greater angle when they are 

 used in immersion contact with the under surface of the 

 slide. — Eng. Mech. and World of Science. 



A New Method of Counting the White Corpuscles. 



Kourloff (Vratch) has devised a new method for count- 

 ing the white corpuscles of the blood, which has been test- 

 ed by him in conjunction with Solovieff : Four or five 

 cubic millimeters of blood are drawn directly from the 

 finger into a graduated mixing pipette, and immediately 

 blown upon the side. Two dry cover-glass specimens are 

 now'prepared and are stained in the usual way, and their 

 area measured by means of a network of lines. The white 

 cells are then counted on both cover-glasses and the area 

 examined is measured by means of the movable stage and 

 Ehrlich's diaphragm. From the figures thus obtained the 

 number of cells in the whole specimen is calculated, and 

 from this result the number of white cells per cubic mil- 

 limeter is determined, the volume of blood used being 



