THE MICROSCOPE. 15 



Diffused daylight is, if possible, employed, and is 

 reflected, by means of a movable mirror (m) below 

 the stage, up through the object and through the 

 tube of the microscope to the eye of the observer. 

 This is termed viewing an object by transmitted 

 light. Occasionally, especially when comparatively 

 large and opaque objects and low magnifying powers 

 are to be employed, the former are viewed by the 

 light which is reflected from their surface, whilst 

 that from the mirror is cut off. In order in such 

 cases to concentrate as much light as possible upon 

 the object, a bull's-eye condenser is employed. It is 

 only in viewing such preparations that the binocular 

 microscope offers any material advantage inhistology. 

 The lenses form the essential part of the micro- 

 scope, and are so arranged that one set of them, 

 which is placed at the lower end of the tube, pro- 

 duces a magnified, inverted image of the object at 

 the upper part. The image thus produced is viewed, 

 and at the same time still further magnified, by a 

 lens at the top of the microscope tube. This lens 

 (e\ placed close to the eye of the observer, is fixed 

 in the eye-piece or ocular ; but this part includes an- 

 other glass (the field-glass,/) situate below the first 

 and fixed in one piece with it, having for its object 

 the collection of the more divergent rays transmitted 

 by the lower set of lenses, and serving also to lessen 

 or obviate any chromatic aberration which might 

 otherwise be produced. The whole eye-piece (oc) 

 thus composed is made to slip in at the superior 

 aperture of the microscope tube. The lower set of 

 lenses form an achromatic combination which, from 

 its situation near the object, is termed the object-glass 

 or objective, and it is upon the perfection of its con- 

 struction that the useful ness of the microscope mainly 

 depends. It is seldom necessary to have more than 

 one ocular of medium strength in use, but at least 

 two objectives of different magnifying powers are 

 essential for histological work. One of these, which 

 in subsequent pages will be spoken of as the low power, 



