16-4 



THE MICROSCOPE. 



of illumination is the steady flame of a paraffin lamp, 

 with a flat wick (fig. 96). Collins's Bocket Lamp, with 

 bull's-eye condenser mounted on a stem, so as to be 

 adjustable at any height, is a suitable form of lamp. 

 Whatever be the source of light, the objects hould on 

 no account be over-illuminated : a flood of light mars 

 the image, and spoils the performance of the object- 

 glass. 



For viewing opaque objects, or whole insects, the 

 eletra of a beetle, etc., the light must be thrown down 



or condensed upon it, by 

 the condensing or bull's- 

 eye lens ; or by Beck's 

 parabolic side- silver re- 

 flector, placed at a pro- 

 per angle to the source 

 of illumination. 



For examining par- 

 tially opaque minute ob- 

 jects, as the Podura-scale, 

 under high-power mag- 

 nification, the vertical 

 illuminator is useful. If 

 the object is a small por- 

 tion of a dissected animal 

 or plant, or a patholo- 

 gical specimen in a fluid 

 medium, the microscope 

 should be employed in 

 the vertical or upright 

 position. The object 

 should be covered with a thin cover-glass, to prevent 

 the escape of the fluid, which, should it run over, might 

 damage the stage and its mechanical movements. 



Test for Illumination. Dr. C. Seiler recommends the 

 human blood corpuscle as the best test of good illumina- 

 tion. He prepares the object in the following manner : 

 Take for the purpose a clean glass slide of the ordinary 

 kind, and place near its extreme edge a drop of fresh 

 blood drawn by pricking the finger with a needle. 

 Then take another slide of the same size, with 



FIG. 96. Collins's Bocket Lamp. 



