THE WEBSTER CONDENSER. 



the stage, a ^-inch or ^-inch objective of wide angle 

 forming a very useful condenser. 



A substage condenser consisting of a plano-convex lens 

 of j-inch focal length was used by Wollaston for illu- 

 minating the object upon the stage ; this was improved by 

 Goring, but still did not satisfy the requirements of micro- 

 scopists. In 1 840 the achromatic condenser was introduced, 

 and for many years afterwards continued to be made as a 

 low-angle objective, adapted in various ways to the stage 

 or substage. Whether 

 achromatic light for 

 illuminating purposes 

 is absolutely necessary 

 is a question we cannot 

 discuss here. 



Messrs. Beck, Ross, 



and Powell and Lea- 



land, together with 

 other makers, have pro- 

 duced at various in- 

 tervals modifications 

 of the achromatic con- 



FIG. 41. 



denser ; but a departure from the usual custom was made in 

 1865 by Mr. Webster, who communicated a drawing of his 

 instrument to the pages of 'Science-Gossip/ and which 

 may be seen in Fig. 41. The diaphragm with which it was 

 furnished may be seen in Fig. 42. 



Mr. Collins's " Webster " condenser is shown in Fig. 43, 

 and is a form extremely well-liked by all practical micro- 

 scopists ; it is made so that the diaphragm may be carried 

 quite close to the lens, and this can be further improved, 

 when dark-ground illumination is required, by placing a 

 circular piece of black paper on the lens itself. It can be 

 used with ease in combination with the polariscope. 



F 



