42 THE MICROSCOPIST. 



for condensing the light upon the object. The best instruments 

 employ an arrangement of achromatic glasses, similar to the 

 object-glasses, but its value is scarcely equal to its cost. The 

 Wollaston Condenser is a short tube, in which a plano-convex 

 lens of three-fourths of an inch focal length, with its flat side 

 towards the object, is made to slide up and down. Dr. Wollas- 

 ton employed a long tube with a stop between the lens and the 

 mirror, but Dr. Goring found it better to have the stop be- 

 tween the lens and the object, and a little out of the axis of 

 the lens. 



A substitute for the achromatic condenser is found in Mr. 

 Varley's dark chamber. This is sometimes preferable to the 

 Wollaston Condenser, as the light is not decomposed by pass- 

 ing through a lens. 



c, Fig. 13, is a plate of brass adapted to the stage, in which 

 is a short tube having a diaphragm or stop, a, whose aperture 



is equal to what can be viewed by the microscope, and no 

 larger. Below is a sliding tube, 6, with an aperture rather 

 larger than that at a. This last can be moved up and down 

 until the light at a is of the greatest intensity. The aperture 

 at a is always in proportion to the object-glass employed. 



Condensers for oblique illumination. As the lines on some 

 test objects require an illumination at a considerable angle from 

 the axis of the microscope, various plans have been suggested 

 for the purpose. The most simple mode is to turn aside the 



