EXAMINATION OF MILK 



193 



well 



mixed and 

 is then well 



I.C.C. 



almost fill the tube. The two fluids are 



centrifugalised for 10 minutes. The cream 



broken up by a clean glass rod, to disentangle 



leucocytes carried to the surface, and the mixture 



centrifugalised for an additional 5 minutes. All 



the fluid is then removed down to the 1 c.c. mark, 



great care being taken not to disturb the deposit. 



This can be conveniently and readily done by means 



of a fine glass tube connected to an exhaust pump. 



Theoretically, all the leucocytes present in the 



original 1 c.c. of milk are now present in the 1 c.c. 



of fluid. The leucocytes are thoroughly well mixed 



(with a wire), and distributed through the 1 c.c. 



A sufficient quantity is placed on the ruled 



squares of the Thoma-Zeiss apparatus, and the 



cover-glass put on. The number of leucocytes is 



counted in a number of different fields of vision, 



moving regularly from one field of vision to another. 



The diameter of the field of vision is ascertained 



before counting by drawing out the microscope 



tube until an exact number of sides of the squares 



spans a diameter of the field of vision. 



The number of leucocytes per cubic m.m. 



56,000 7/ 



— 2 — > where y = the average number of leucocytes per 



field of vision, d — the number of squares which just spans the 

 diameter, d is determined once for all by marking the micro- 

 scope draw tube so that ouly 20 fields have to be counted, 

 and the figures substituted in the formula.^ 



Doane-BucMey Method. — 10 c.c. of milk are centrifugalised 

 for 4 minutes in a graduated centrifuge tube running at about 

 2000 revolutions per minute. The cream is removed with a 



Fig. 9. 

 of milk = 



' It has been urged as a criticism that this counting procedure makes the 

 method unnecessarily difficult. The counting by field of vision is vastly more 

 accurate than by squares. If the former is done some formula is necessary, but 

 if d is once determined and a line scratched on the microscope tube, the mathe- 

 matical labour is trivial. In the microscope chiefly used by the writer, d was 



always 6, and the formula became 



56,000 2/14,000 



y. For ordinary routine 



11x86 99 



work — number = 1402/ "^^^^ then sufficiently accurate, or even more simply, just 

 7 times the number of leucocytes in the total 20 fields counted. 







