274 HIGH POWER MICRO-PROJECTION [Cn. IX 



2^ to 3 cm. (t to i J/2 in.) from the focus of the converging cone. 

 The concave lens will render the converging light approximately 

 parallel, and this cylinder of light is small enough to enter the 

 substage condenser. By a small manipulation of the screw of the 

 substage condenser bringing it slightly nearer the specimen or 

 slightly farther from it the most brilliant screen image can be pro- 

 duced. A slight change in the position of the substage condenser 

 often works wonders. 



FIG. 151. RELATION OF THE APERTURE OF THE LIGHT FROM THE COX- 

 DENSER TO THE APERTURE OF THE OBJECTIVE. 



(From Nelson, Jour. Roy. Micr. Soc.). 



A The cone of light from the condenser just fills the aperture of the objec- 

 tive (B). 



B Back lens of the objective entirely filled with light. 



C The cone of light from the condenser is not great enough to fill the aper- 

 ture of the objective (D). 



D Back lens of the objective lighted by the condenser (C). 



The dark ring shows the aperture of the objective not lighted by the con- 

 denser. 



B. Rendering the diverging cone of light approximately parallel 

 by the use of a convex lens. If a convex lens is placed in the path 

 of the diverging cone at its focal distance from the focus of the main 

 condenser, the light will be rendered parallel. In order to have a 

 cylinder of light of the right size to enter the substage condenser a 

 convex lens of the proper focal length and diameter must be used. 

 Trial lenses arc excellent. Those of 10 and 20 diopters (100 and 

 50 mm., 4 to 2 in. focus) arc excellent for the main condenser with 

 a focus of 150 to 200 mm. (6 to 8 in.). The microscope must be 

 put in such a position that the trial lens in the fork before the sub- 

 stage condenser shall be at its focal distance from the focus of the 

 main condenser. The diverging cone of light will be made approxi- 



