122 



ELEMENTS OF MICROSCOPY. 



means of procuring illumination usually consist of a single concave, 

 or a double plane and concave, mirror, upon an axis fixed below 

 the stage. This, however, is usually of very inadequate size in 

 small stands. This is a serious error, since the amount of light 

 required to illuminate an object is by no means dependent on the 

 size of the support upon which the object rests — />., the stage. 

 We shall suggest hereafter methods of overcoming this difficulty. 

 About the stage itself not much need be said. It consists of a 

 support, which should be absolutely level, fixed accurately at right 

 angles to the body of the microscope, or at least to the axis of the 

 optical tube, and so rigid as not to " spring " with any reasonable 

 amount of pressure. 



BJtHai/iii 



The simple form of the '' Nelson " microscope, of which a cut 

 is given, shows the various features above referred to in their best 

 and most convenient form. The object of the somewhat un- 

 familiar shape of the stage is that, when working with high 

 powers, the distance of the object from the lens may be judged 

 by gently raising the slide with the finger, thus avoiding the risk 

 of serious injury to both slide and objective, which is apt to occur, 

 even in experienced hands, when sight alone is relied upon. 



