THE MICROSCOPE. 91 



The stand should be solid and firm, without springiness or 

 "give" in its various parts. The base should be heavy so as 

 to prevent its easy overthrow. It is frequently convenient to 

 have the instrument so arranged that it may be inclined ; but 

 if an instrument with short tube and low body be procured 

 and fitted with a camera admitting of use in a vertical posi- 

 tion, inclination is rarely necessary. 



The accessories which are necessary for the biologist are ex- 

 tremely few. First among them comes the camera lucida or 

 other means of seeing the object and the point of the pencil 

 at the same time. The simplest form consists of a bit of 

 thin glass so mounted near the eye-piece of the microscope 

 that the eye can see the point of the pencil through it, and at 

 the same time the image coming through the eye-piece is re- 

 flected by it to the eye. In other forms prisms of various 

 shapes replace the thin glass, or a very small metallic mirror 

 is employed. A prism properly mounted forms the most satis- 

 factory camera. 



Occasionally, in differentiating certain structures in the 

 living animal, a polariscope is useful. This consists of two 

 prisms of Iceland spar properly prepared and placed, the one 

 below the object and between it and the mirror, the other, 

 either in connection with the objective or the eye-piece, be- 

 tween the object and the eye. When either of these is 

 revolved around the axis of the instrument, many structures 

 are seen to present different colors which vary as do the rel- 

 ative positions of the two prisms. 



Other accessories such as mechanical stages, spot lenses, 



