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. 1 



Two types of these rulings are shown in Figs. 129 and 130. 



FIG. 129. Haemacytometer Cell. 

 Xi$. Turk rulings. 



FIG. 130. Haemacytometer Cell. 

 X 1 5. 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 haemacytometers may be obtained from Max Levy, Phila- 

 delphia, Pa. Bausch & Lomb Optical Co., Rochester, N. Y. 



