ZOOLOGY AND BOTANY^ MICROSCOPY, ETC. 



295 



MICROSCOPY. 



A. Instruments, Accessories, etc. 

 (3} Illuminating: and other Apparatus. 



Opacimeter for Standardizing Bacterial Emulsions. — Lambert, 

 Yles and De Watteville {C.R. UAcad. Sci., 1919, 168, No. 15, 797-9. 

 Vies has previously suggested that the standardization of a bacterial 

 emulsion might be effected by measuring the loss sustained by a 

 luminous ray in traversing this emulsion. The present writers point 

 out that in the construction of a bacteriological opacimeter certain 

 properties of the emulsions necessitate the use of a monochromatic 

 light with a fixed wave length, whilst bacteriological technique requires 

 the use of absolutely sterile vessels. 



Their opacimeter is a photometer made by two luminous rays in 

 juxtaposition. Whilst one traverses the emulsion, the intensity of the 



other can be varied at will to enable the two planes of light to be 

 equalized in the observer's ocular field. The most convenient means of 

 weakening the second ray is the use of progressively darkened photo- 

 graphic plates varying from zero to total opacity, previously calibrated 

 by spectrophotometric measurements. 



The apparatus (see figure), constructed in 1916 in the Sorbon'Physical 

 Research Laboratory, and used in the French Army at the anti-typhoid 

 vaccination laboratories, was planned as follows : — S is the source, 

 100-candle power nitrogen lamp in a metal shield which lets the ray 



