368 



SUMMARY OF CURKENT RESEARCHES RELATING TO 



this purpose. It is necessary to use a low-power object-glass first in 

 order to bring the image of the source of light to the centre of the 

 field of view before proceeding to examine the object with higher 

 powers. For the examination of colloidal solutions it is advisable to use 

 small chambers on a glass slide, which after filling are covered with a slip. 

 The depth of the chamber can be • 1, "2 or U • 3 mm., as may be required, 

 and the diameter l(i mm. When the liquid to be examined is placed 

 on the slide and covered with a slip, it dries up quickly, but in these 

 simple chambers it can be kept for hours in good condition. AVheii 



Fig. 60. 



large quantities of liquids, or their mixture with other liquids, are to 

 be examined, the arrangement shown in fig. 61 should be adopted. The 

 liquid is forced to flow from one vessel to the other through a space 

 in the centre of the chamber on the slide. The latter is very similar 

 to those already described. For observing bacteria glass slides which 

 have a small concavity ground in them are suitable. As a»rule, 

 however, the ordinaryi slides are all that is required. A drop of 

 liquid containing part of the pure culture is placed on the slide covered 

 with a slip. The flagella of several kinds of bacteria — for instance. 

 Spirillum volutans — are distinctly seen by means of the reflecting 

 condenser. It is, however, advisable to employ some means to reduce 

 the extremely rapid movements. It seems also desirable that an im- 

 bedding material, the diffraction of which is very different from the 



