CHAPTER VII. 



THE BACTERIOLOGICAL MICROSCOPE. 



THE instruments sometimes in use in biological and pathological 

 laboratories are not sufficient for the study of bacteria. It is 

 absolutely essential for the examination of such minute objects that 

 the microscope should be equipped with an objective of sufficiently 

 high magnifying power and with a special illuminating apparatus, 

 while the mechanical arrangements of the stage must admit of the 

 examination of plate-cultivations. It would not be within the scope 

 of this work to give a detailed account of the mechanical arrange- 

 ments and optical principles of the microscope. These matters are 

 fully dealt with in special works on the subject,* but sufficient will 

 be said to afford assistance in the selection of a suitable instrument, 

 and to explain the improvements in the microscope which have been 

 such an aid in bacteriological investigations. 



A magnified image of an object is the result of the change 

 produced in the direction of rays of light which are made to pass 

 through lenses. This alteration in the course of the rays is known 

 as refraction. A ray of light passing from a larer into a denser 

 medium is refracted towards a line drawn perpendicularly to the 

 surface of the latter. A ray of light passing through air and 

 impinging on water will not pass on in the same direction, but will 

 be refracted towards a line drawn perpendicularly towards the 

 MirfiK-e of the water. If the ray pass into glass instead of water 

 a greater refraction will take place, and if it pass into diamond the 

 bending in its course will be still greater (Fig. 11). 



The sines of the angle of incidence and refraction of different 



substances have a constant ratio to each other, which is known 



he index of refraction, and this is determined for different 



substances by the refraction produced by the passage of rays from 



;i vacuum. Thus the index of refraction for flint glass is about 1*6, 



* Carpenter : The Microscope. Nageli and Schwenderer : The Microscope 

 in Theory and Practice. 



65 n 



