158 SELECTION AND USE 



angle lias greatly enlarged the scope of usefulness of this little 

 piece of apparatus. 



The Wenham prism consists of a small right-angled prism of 

 crown glass, which is " patched " on to the under side of the 

 slide by means of a little glycerine, oil of cloves, or any similar 

 fluid. In the figure, b is the slide and 

 . ~ -. ^ cover, and a is the prism. Rays of 

 ' light* as shown by the dotted lines, 



^ sen ^ through the face of the prism, 

 pass straight to the upper surface of 

 Fig. 47. the cover, and if a dry objective be 



THE WENHAM PRISM. used, they are reflected down again on 

 to the underlying objects in balsam, 



which appear brilliantly illuminated, as if beneath a speculum. 

 An immersion object-glass prevents the covering glass from 

 acting as a speculum, and light emerges beyond what would 

 otherwise be the critical angle, thus affording very oblique illu- 

 mination, which, with suitable objectives, enables us to resolve 

 the most difficult tests. 



If the prism is used with dry mounted objects, total reflection, 

 instead of occurring from the cover, takes place from the upper 

 surface of the slide itself, and all is total obscurity except where 

 some object, such as a butterfly's scale or a diatom, is in inti- 

 mate contact with the slide, and then these objects allow the 

 transmission of light, and appear curiously distinct in a jet- 

 black field. 



This illuminator requires no fitting to adapt it to the micro- 

 scope. It is merely stuck on the under surface of the slide 

 with a little glycerine ; the slide is then placed on the stage, 

 the object found, and the position of the prism adjusted by the 

 finger. 



It has been objected to this method of attaching it, that when 

 the microscope is inclined, the prism is apt to slide down. This 

 will not occur if the quantity of liquid used be very small. It 

 is not difficult, however, to arrange a very thin plate of metal 

 with a square hole in the centre, the metal which originally 

 filled the hole being bent down so as to form an ear or clip 

 at each side of the opening. These clips grasp the prism, 

 which, being cemented into this frame, may be laid on the 



