296 SUMMARY OF CUKRENT RESEARCHES RELATING TO 



out in two directions at right angles. A lens l^ throws a number 

 of parallel rays through two windows in the opposite walls of a copper 

 water-tight vessel B filled with water, in which the cylinder of emulsion 

 is immersed. (The water serves to lessen the phenomena of refraction 

 on the surface of this receptacle.) From the exit the rays traverse a 

 coloured screen e-^ (/. = 620 fi), are intercepted by a prism of total 

 reflection p^ and a lense I2, and are thrown on to a glass cube c ; this is 

 formed by two rectangular prisms joined at their hypotenuses, one of 

 which, partially silvered, throws the rays on to the microscope m. The 

 other rays, reflected by p^ and filtered by the coloured screen e.2» ^^'6 

 intercepted by an objective I3, and striking through the glass cube at its 

 non-silvered part, come into juxtaposition with the other rays within 

 the ocular field of the microscope. Before crossing the objective I3 the 

 second rays traverse a progressively darkened photographic plate e,. 

 This is mounted on a glass cylinder revolving on its own axis by means 

 of the hand-gear Lj outside the apparatus. It has a photographic scale 

 dividing the screen into 100 equal parts. Tiie characteristics of the 

 objective /g are such that the reflection of these divisions appears at the 

 level of the silvered surface of the glass cube, so that it appears in the 

 ocular field identically with the two planes of light. The observation 

 for the equality of the planes and the reading of the graduations may 

 therefore be done together. In order to get rid of any preconceived 

 ideas when making the reading, a small movable shutter permits of the 

 graduations being shut out at the wish of the operator by means 

 of a hand-gear h^. 



Under the heading emulsions the authors state that very dilute 

 emulsions should be contained in cylinders, 10 cm. diameter, whilst 

 very concentrated emulsions should be put in glass tubes of 3| mm. 

 Both receptacles are easily sterilized in the ordinary oven or autoclave. 



The authors also describe the method of graduating the screen which 

 weaken the rays, as follows :— The graduation marks of the progressive 

 screen are definite and must be gauged empirically. The gauging con- 

 sists in constructing a graph connecting these divisions to the charac- 

 teristic chosen as representing the quantity of bacterial substance in the 

 emulsion — e.g. the dry weight of bacteria per cubic centimetre. The 

 technique is briefly as under : — 



The emulsion chosen as type is divided into two portions. One of 

 these, say 80 c.cm., is centrifugalized at 7000 revolutions, the deposit 

 washed with distilled water, re-centrifugalized, dried at 110° C. to 

 constant weight, and weighed. This gives the dry weight p in milli- 

 grams per cubic centimetre of the type emulsion. The other part, 

 diluted 2, 8, 4, . . . times, gives a series of derived emulsions whose dry 

 weights are obviously p/2, p/S, p/4:. . . . The whole series is sub- 

 mitted to opacimetric measurements. A graph is then constructed 

 having in abscissa the opacimetric measurements of these dilutions, and 

 in ordinate the corresponding dry weights. 



It might be useful to make the same experiment with several type 

 emulsions, and to construct a mean curve for the bacterial species 

 under consideration. In conformity with bacteriological usages, it may 

 sometimes be necessary to express the standards, not in dry weights. 



