206 
ELEMENTARY CHEMICAL MICROSCOPY 
and the difficulty of properly cleaning the slides without event¬ 
ually injuring the rulings. 
When counts are required of very minute objects such as 
bacteria, mold spores, yeasts, finely divided particles in suspen¬ 
sion, and the like, cells having exceptionally fine rulings are 
essential; recourse is then had to haemacytometers (blood count¬ 
ing cells). These cells are generally o.i mm. deep and are ruled 
in 0.0025 sq. mm.i 
Two types of these rulings are shown in Figs. 129 and 130. 
P'lG. 129. Hsemacytometer Cell. Fig. 130. Haemacytometer Cell. 
X15. Turk rulings. X15. Zappert-Neubauer rulings. 
When it is desirable to cover a definite area on' the object 
slide it is far better to employ a micrometer disk-diaphragm 
properly calibrated and inserted into the eyepiece or to cut a 
square opening in a disk of dull black paper, thin card, metal or 
blackened mica, and drop the disk into the proper eyepiece by 
removing the eye-lens and allowing the disk to rest upon the 
diaphragm of the eyepiece. The proper size of opening is ascer¬ 
tained by eyepiece and stage micrometers, and a square hole of 
this calculated size is cut in the paper and the perforated disk 
is inserted in the eyepiece. The final adjustment is then made 
with the draw-tube. 
A more convenient and more economical procedure is as 
1 American made ha;macytometers may be obtained from Max Levy, Phila¬ 
delphia, Pa. Bausch & Lomb Optical Co., Rochester, N. Y. 
