Oct. 8,1921 Microscopic Study of Bacteria in Cheese 95 



In order to determine the effect of the embedding process upon the 

 cheese, small measured cubes of cheese were subjected to the routine 

 procedures. Only a slight shrinkage was found, indicating that the 

 volume of embedded cheese when examined is approximately the same 

 as that of the fresh sample. 



MICROSCOPIC EXAMINATION 



The preparations were examined with an oil immersion lens and a 

 high power ocular, the most satisfactory combination being a 1.9-mm. 

 fluorite objective with a numerical aperture of 1.32. Where a thick 

 coverslip was used it was necessary to have a 3-mm. apochromatic 

 objective with a numerical aperture of 1.4. Greater depth can be secured 

 with compensating oculars than with the ordinary Huygenian oculars. 



The method, although at first used only for determining the types of 

 organisms present in the samples and as a check on the usual plate 

 method, was found useful as a means of determining the number of 

 organisms present. In order to make such a computation the micro- 

 scope was so standardized as to allow an estimate of the number of 

 organisms per gram when only a small amount of the original section 

 was examined. This computation is similar to that used in the direct 

 method of counting bacteria in milk described by Breed and Brew (2). 

 This was accomplished by measuring both the diameter of the micro- 

 scopic field and the thickness of the section from which the amount of 

 cheese actually seen in each field examined was determined. Knowing 

 the volume and specific gravity of the cheese examined, the total number 

 of organisms per gram can readily be computed. With the diameter 

 of the field measuring 0.14 mm. (140 fx), the microtome so adjusted as 

 to cut sections of a thickness of 0.005 nim. (5 ij.) , and a specific gravity of i, 

 the amount of cheese examined per microscopic field would be 1/13,000,000 

 gm. — that is, each organism observed in a single microscopic field repre- 

 sents 13,000,000 per gram. 



This factor may be computed by the following formula, in which any 

 measure may be substituted : 



1,000 , . , 

 — -2 — = factor per gram. 



In the above formula, 



r = the radius of the field examined in millimeters as determined by 

 actual measurement. 



a = the thickness of the section in millimeters. 



b = the specific gravity of the cheese. 



The radius of the field, as has been stated, is determined by measure- 

 ment with a stage micrometer and varies with the magnification and with 

 the tyoe of ocular used. However, it was found advisable to adjust the 



