LIVING MICRO-ORGANISMS 245 



Magnifications of about 1000 are required for bacteria, 

 but 500 times is enough for yeast cells, and even lower 

 magnifications are ample for some moulds. The most 

 useful lenses are the T V' or T V" oil-immersion with a 

 medium eyepiece, a J" with a x 6 or x 8 eyepiece, or a 

 J" apochromat with a x 8 or Xl2 eyepiece. The dry 

 lenses have the advantage that they do not disturb the 

 cells by communicating movement through a film of oil, 

 but they require relatively longer exposure than the 

 immersion objectives. 



The high-power dark ground illuminators are useful 

 for living micro-organisms, particularly for the rapid 

 diagnosis of such forms as Spirochceta pallida, etc., which 

 otherwise require special staining methods for detection ; 

 a reference has already been made to their use in cine- 

 matography. The living cells in a mount of yeast are 

 shown with a very bright outline, while dead or shrivelled 

 cells can be rapidly picked out by their shining appear- 

 ance. An arc lamp is required to allow of exposures of 

 lyV", but the more sluggish or non-motile organisms can 

 be photographed with a less powerful illuminant, a 

 Nernst for example. Two photomicrographs of living 

 organisms on a dark field are given on Plate 36. 



It is very easy to produce alternative or even false 

 appearances in a picture that depends so largely on 

 diffraction for the display of its detail, and, in fact, the 

 colourless transparent protoplasm of bacteria or yeast 

 when photographed by transmitted light can be repre- 

 sented by dark structure on a light ground, or by light 

 on a dark ground. The former is more frequently used, 

 and Plate 36. B is an example, but the white image gives 

 a much better idea of the solidity of the cell (Plate 36. A 

 and E). Either image can be obtained at will by a careful 

 adjustment of the condenser and mirror, or even by a 

 slight alteration in focus, and that which shows best the 

 structure or detail required should be chosen. A slight 



