174 Transactions of the Society. 



present in various preparations, so that when one is known the 

 other can be calculated. It does not, however, enable one to 

 determine the actual weights of the material in which the counts 

 have been made. One can immediately ascertain the weight of 

 any counted number of lycopodium spores if one knows the 

 average weight of one spore, or, as it is more conveniently expressed,, 

 the number of spores per milligramme of lycopodium. Then, since 

 the lycopodium and other material have been mixed in known 

 proportions by weight, the corresponding weight of the other 

 substance can be found by a simple calculation. This weight of 

 material contains the counted number of characteristic particles, 

 and from this can be calculated the number of such particles per 

 milligramme. Hence for each substance there is a definite number 

 representing the countable particles per milligramme, and these 

 figures can be used as a means of characterizing or of standardizing 

 the material. Such figures are not always suitable for calculating 

 the amount of the substance present in mixtures, for which purpose 

 it is better to obtain a special number by counting a mixture of 

 known composition containing the same ingredients, but not 

 necessarily in the same proportions as the material to be examined. 

 The figures representing pure substances form a useful check upon 

 those obtained by counting standard mixtures, but cannot safely be 

 used in their place. 



Determination of the Number of Spores per 

 Milligramme of Lycopodium. 



The number of spores per milligramme of lycopodium has been 

 shown to average 94,000, a figure which was obtained by weighing 

 accurately about 0*1 grm. of lycopodium and mixing it with a 

 definite weight, about 10 to 12 grm., of olive oil or mucilage 

 of tragacanth. A clean microscopic slide and cover-glass were 

 then weighed, and a drop of the suspension was mounted on the 

 slide, which was again weighed, thus giving the weight of the 

 suspension on the slide. Twenty fields were then counted accord- 

 ing to a pre-arranged plan similar to that advocated for quantitative 

 microscopy in general. The total counted number of spores- 

 multiplied by the area of the cover-glass and divided by the area 

 of twenty fields gives the number of spores under the cover-glass. 

 From this number, the total weight of suspension and the weight 

 of suspension mounted on the slide, one obtains the number of 

 spores per milligramme. 



The following example will show how the result is obtained : — 



Weight of lycopodium .....= 0*1102 grm, 

 Weight of suspension (oil and lycopodium) . = 9*8560 ,, 

 Weight of suspension on the slide . . . = 0*0276 „ 



