160 SELECTION AND USE 



button," as it has been called, is connected to tlie tinder side 

 of the slide by means of water, glycerine, or oil of cloves, its 

 sides being grasped by a simple kind of open clip attached to 

 the sub-stage. This illuminator is complete in itself, and re- 

 quires no supplementary condensing lens ; the obliquity is 



Figs. 48. 



simply obtained by swinging the ordinary mirror sideways, 

 and by this means Amphipleura pellucida, mounted in balsam, 

 can be at once resolved. 



The Woodward Illuminator. This method of mount- 

 ing and arranging a right-angled prism for the illumination of 

 balsam-moulted objects, was described by Col. Dr. Woodward 

 in a paper read before the Eoyal Microscopical Society, June 6, 

 1877. It consists of a prism of glass, the apex of which has 

 been truncated. This prism is cemented to the truncated apex 

 of a similar prism of brass, the long side of the glass prism 

 being upward, and connected with the under surface of the 

 object-slide by suitable liquid (glycerine, oil of cloves, etc.) 

 The homologous side of the brass prism is downwards, and 

 slides in a holder which is supported by the sub-stage. To 

 this brass prism are attached two arms, which serve to support 

 a thin screen at some distance below the stage. The screen 

 is parallel to that face of the glass prism that receives the light, 

 and is pierced with a pin-hole, which permits the passage of a 

 minute beam of sunlight. With this apparatus, Col. Dr. 

 Woodward secured very fine results. 



Tolles' Illuminating Traverse .Lens. The most 

 perfect arrangement for allowing a beam of light to reach, 

 without refraction, a suitably immersed object, is the illumina- 

 ting traverse lens of Mr. Tolles. The engraving, Fig. 51, and the 

 description, which is in Mr. Tolles' own words, are from the 

 Journal of the Royal Microscopical Society. 



