52 MANUAL OF HISTOLOGY. 



-ituated within the large square, it becomes easy, by a simple 

 equation, to find the number in a cubic millimetre. A single 

 count, however, exposes to sources of error, and in order to 

 approach more nearly to exactness, I have uniformly counted 

 the number contained in the large square in five different por- 

 tions of the field (sometimes ten), and have taken a mean of the 

 whole number of counts as the standard. 



The computation is as follows : The glass cell on the slide 

 is | mm. deep. The eye-piece micrometer marks off mm. 

 square, therefore the count of red corpuscles (or white, as the 

 case may be) must indicate the number contained (in the dilu- 

 tion used) in mm. cube. But | mm. cube is ^ of a c.mm., 

 therefore the number counted must be multiplied by 125 ; and 

 the blood was diluted by adding 250 parts of fluid to 1 of blood 

 (2 c.mm. to 500 c.mm.), therefore the product above obtained 

 must be again multiplied by 251 to get the number of corpus- 

 cles in a c.mm. of pure blood. Instead of multiplying twice, a 

 single multiplication by the product of 125 x 251, 31,375, will 

 give the same result." 



This method should, theoretically, be absolutely accurate, but there are vari- 

 ous errors which will unavoidably creep in. First of all, the tubes should be 

 verified as to accuracy. This has been done for me at the Winchester Observa- 

 tory, of Yale College, by Leonard Waldo, Esq., the astronomer in charge. My 

 larger glass tube is slightly different in shape from the one here represented, and 

 is marked so that the line at \ indicates a capacity of 500 cubic millimetres 

 (0.5005 grammes of distilled water at 26.4 C.). The cubical contents of the 

 reservoir from the "point to the line i = 0.2425 + 0.008 = 2505 grammes = 

 250 c.mm., approximately. Accordingly, the marks i and \ indicate i and | 

 a cubic centimetre, within a. limit of error so small as to be practically insensi- 

 ble. The smaller glass tube, which is capillary, is marked 2, 2$, 4, and 5. 

 The level 5 indicates a capacity of 5 c.mm. The capacity between the pointed 

 extremity and 2 is 2 c.mm., less jfo c.mm. ; the space between 2 and 2| con- 

 tains .55 c.mm. ; the space between 2 and 4 contains 1.45 c.mm. ; the space 

 between 4 and 5 contains exactly 1 c.mm. (Waldo). The determination of these 

 capacities was made by using distilled water, and comparing the weight, when 

 filled to the various levels, with the same tube after careful drying. 



These estimates are given to show one of the errors which may be met 

 with, and that an instrument, before using, should be verified by some one who 

 has special means for determining capacities of this kind. My eye-piece mi- 

 crometer was made for me by Rogers, of Cambridge, and the entire field was 

 subdivided into squares, so that every portion of it may be counted without 

 moving the slide. My method has been practically the same as that of Dr. 

 K3yes, except that I prefer diluting with one thousand parts of the diluent, 



