34 THE MICROSCOPIST. 



tures are more or less occupied by stops, or solid disks, so 

 as to leave but a ring of space for light to pass through. 

 The effect of these annular diaphragms is similar to an 

 apparatus for oblique illumination. 



The Webster condenser is similar in its optical parts to 

 the Kellner eye-piece, and is provided with a diaphragm 

 plate, with stops for oblique illumination, as well as a 



Webster's Condenser, with Graduating Diaphragms. 



graduating diaphragm for the regulation of the central 

 aperture. This is a most useful accessory. (Fig. 12.) 



Oblique Illuminators Certain fine markings on trans- 

 parent objects can scarcely be made out by central illumi- 

 nation, but require the rays to come from one side, so as 

 to throw a shadow. Sometimes this is well accomplished 

 by turning the mirror aside from the axis of the micro- 

 scope, and sometimes by the use of one of the condensers 

 referred to above. Amici's prism, which has both plane 

 and lenticular surfaces, is sometimes used on one side and 

 under the stage, in lieu of the mirror. For obtaining 

 very oblique pencils of light the double hemispherical con- 

 denser of Mr. Reade has been invented. It is a hemi- 

 spherical lens of about one and a half inch diameter, with 

 its flat side next the object, surmounted by a smaller lens 

 of the same form, the flat side of which is covered with a 

 thin diaphragm, having an aperture or apertures close to 



