THE MICROSCOPE. 



45 



b. Single prism. Mr. Bryson of Edinburgh has recently 

 fixed a small rectangular prism in a simple support, that 

 can be readily clipped on the eye-piece of the microscope. 



FIG. 38. Chevalier's camera lucida, with Hartnack's microscope. 



It acts like p in Fig. 38, gives as sharp an image as 

 Chevalier's, and is far less expensive. The observer keeps 

 his head in a vertical position, and makes the drawing on a 

 suitable support placed vertically or slightly inclined ten 

 inches in front of the eye. The observer should, therefore, 

 sit at the left side of the microscope when it is directed 

 towards the window, so that he may not obstruct the illumi- 

 nation of the mirror, nor get the direct light from the 

 window into the eye that looks at the paper. The position 

 of the hand in drawing is not quite so convenient as when 

 it rests on the table, but really a camera is not very often 

 employed, and the cheapness of the apparatus is a com- 

 pensation. 



c. Parallel plate reflector. A slip of thin glass placed above the 

 ocular at an angle of 45 (r, Fig. 39), has long been recommended in 



