170 Transactions of the Society. 



Perhaps the greatest difficulty with which one is faced is to 

 devise a simple means of ascertaining the quantity of material in 

 which certain specified particles have been counted by the micro- 

 scope. 



Some of the earlier methods proposed and used {1 to 5*) give 

 only approximate results, with an uncertain range of error, and 

 may lead to erroneous conclusions. Other more reliable methods 

 require either a specially constructed mechanical stage {6 and 7), 

 or a specially made glass slide with squares of a known size ruled 

 over a limited area equal to that of the cover-glass used {8), or a 

 glass slide with a well of a particular depth and squares engraved 

 upon the enclosed area {9). All these methods are of limited 

 application, and are tedious to carry out. 



To bring quantitative measurements within the sphere of 

 regular microscopical practice, one needs a general method applic- 

 able with slight modifications to a wide range of substances, and 

 requiring no specially constructed apparatus. The method should 

 be simple in principle, and reliable within comparatively narrow 

 limits for all percentages of admixture. The use of lycopodium as 

 suggested by the author {12) provides such a generally applicable 

 process which gives results rarely showing an error greater than 

 10 p.c. of the amount to be determined. It is therefore possible, 

 for example, in dealing with an admixture present to the extent of 

 20 p.c. to obtain a result that will lie between 18 and 22 p.c. 

 Kesults showing this order of accuracy are quite equal to many of 

 those obtained by chemical operations, and the careful use of this 

 lycopodium method entitles microscopical quantitative determina- 

 tions of suitable materials to rank on an equality with those made 

 by the use of many well-tried chemical processes. 



Outline of the Method. 



In general outline the method of procedure is as follows : — 

 The nature of the admixture is first ascertained, and a powder con- 

 taining this ingredient and the pure substance in equal proportions 

 is prepared. Of this 50 p.c. mixture a convenient weighed amount 

 (about 0*2 grm. in most cases) is mixed with a weighed quan- 

 tity (about • 1 grm.) of lycopodium spores, and with a suitable 

 volume (about 20 c.c.) of a suspending agent such as mucilage of 

 tragacanth, olive oil or castor oil. 



A drop of the suspension is mounted for microscopical examina- 

 tion and the number of lycopodium spores ajid of characteristic 

 elements of the powder are counted in ten fields selected according 

 to the scheme described below. A second drop of the suspension 

 is mounted and ten fields counted as for the first slide, and the 



* The italic figures in brackets refer to the Bibliography at the end of the 

 paper. 



