THE IMMERSION ILLUMINATOR, 175 



angled prism, connected by a fluid medium of oil of 

 turpentine, or oil of cloves. This, however, was aban- 

 doned for a nearly hemispherical 

 lens, connected with the slide, 

 and which, although a great im- 

 provement, did not reach the 

 point of excellence Mr. Wenham 

 was looking for. Ultimately he FIG. 107. FIG. io7a. 

 adopted a semicircular disc of 



glass of the exact form and size represented in the 

 drawing, fig. 107, being a side view, and fig. 10 7a, 

 an edge view of the same, and having a quarter- 

 inch radius, with a well-polished rounded edge, the 

 sides being grasped by a simple kind of open clip 

 attached to the sub-stage. The fluid medium used for 

 Connecting the upper surface with the slide being 

 either water, glycerine, or oil ; a certain increase of 

 obliquity being obtained by swinging the ordinary 

 mirror sideways. By means of an illuminator of the 

 kind difficult objects mounted in balsam were resolyed. 

 This simple piece of glass, in appearance somewhat 

 resembling the half of a broken button half an inch 

 in diameter, collects and concentrates light in a sur- 

 prising way, and is by no means a bad substitute for 

 the more costly forms of achromatic condenser. It 

 can be used either in fluid contact with the slide, or 

 dry, as an ordinary condenser. 



Mr. Wenham subsequently contrived a small trun- 

 cated glass paraboloid, for use in fluid contact with the 

 slide ; water, glycerine, gum, oil, or other substance 

 being employed as a contact medium. The rays of light 

 in this illuminator being internally reflected from a 

 convex surface of glass, impinge very obliquely on 

 the under surface of the slide, and are transmitted by 

 the fluid uniting medium, and internally reflected from 

 the upper surface of the cover-glass to the objective. 

 To use the reflex illuminator efficiently it must be 

 racked up to a level with the stage. The centre of 

 rotation is then set true by a dot on the fitting, seen 

 with a low power, a drop of water is then placed on 

 the top, and upon this the slide is laid. Minute objects 



