MICROSCOPIC ACCESSORIES. 



35 



its margin. These apertures may be Y-shaped, extending 

 to about a quarter of an inch from the centre. 



If the microscope has a mechanical stage, with rack- 

 work, or is otherwise too thick to permit the mirror to 

 be turned aside for very oblique illumination, Nachefs 

 prism will prove of service. I have also contrived a useful 

 oblique illuminator for this purpose, by cementing with 

 Dammar varnish a plano-convex lens on one face of a to- 

 tally- reflecting prism, and near the upper edge of the 

 other side (at 90) an achromatic lens from a French trip- 

 let. The prism is made to turn on a hinge, so that an 

 accurate pencil of light may fall on the object at any 

 angle desired. 



Dark-ground Illuminators. Some beautiful effects are 

 produced, and the demonstration of some structures aided, 

 by preventing the light condensed upon the object from 

 entering the object-glass. In this way the object appears 



FIG. 13. 



FIG. 14. 



Nobert's Illuminator. 



Parabolic Illuminator. 



self-luminous on a black ground. For low powers this 

 can be easily done by turning aside the concave mirror as 

 in oblique illumination, or by employing Nobert's illumi- 

 nator, which is a thick plano-convex lens, in the convex 



