36 



THE MICROSCOPIST. 



surface of which a deep concavity is made. The plane 

 side is next the object. This throws an oblique light all 

 round the object. A substitute for this, called a spot lens, 

 is often used, and differs only from Robert's in having a 

 central black stop on the plane side instead of a concavity 

 (Fig. 13). A still greater degree of obliquity suitable for 

 high powers must be sought by the use of the parabolic 

 illuminator (Fig. 14). This is usually a paraboloid of glass, 

 which reflects to a focus the rays which fall upon its inter- 

 nal surface, while the central rays are stopped. 



Illuminators for Opaque Objects. Ordinary daylight is 

 hardly sufficient for the illumination of opaque objects, 



FIG. 15. 



Bull's-eye Condenser. 



so that microscopists resort to concentrated lamplight, etc. 

 Gas, paraffine, and camphene lamps, have been variously 

 modified for this purpose, but few are better than the Ger- 



