MICROSCOPIC ACCESSORIES. 37 



man student's Argand lamp for petroleum or kerosene 

 oil, as it is called. To concentrate the light from such a 

 source a condensing lens is used, either attached to the 

 microscope or mounted on a separate stand. Sometimes 

 a bull's-eye condenser is used for more effective illumination 

 (Fig. 15). This is a large plano-convex lens of short focus, 

 mounted on a stand. For such a lens the position of least 

 spherical aberration is when its convex side is towards 

 parallel rays ; hence, in daylight, the plane side should be 

 next the object. But, if it is desired to render the diverg- 



FIG. 16. 



Parabolic Speculum. 



ing rays of a lamp parallel, the plane side should be next 

 the lamp, and rather close to it. The use of this con- 

 denser will also commend itself, when used as last referred 

 to, in microscopic dissection. It will throw a bright light 

 from the lamp directly on the trough, watch-glass, etc., in 

 which the specimen is being prepared. The Lieberkuhn, 

 or a concave speculum attached to the object-glass, and 

 reflecting the light from the mirror directly upon the 

 object, is one of the oldest contrivances for the illumina- 

 tion of opaque objects ; but the most convenient instru- 

 ment is the parabolic speculum (Fig. 16), a side mirror with 



