io8 



PRACTICAL MICROSCOPY. 



vessel and cell should be clearly and sharply delineated, 

 there must be no mistiness or blackness of edge. A dark 



FIG. 98. 



image shows at once that too much of the peripheral 

 pencil has been cut off, the definition of a small aperture 



objective being never equal 

 to one" of large angle. 

 The figure shows a sec- 

 tion of the Horse-chestnut 

 stem (^Esculus hippocasta- 

 num). 



Triplets, such as are 

 shown in Fig. 28, gene- 

 rally break down under 

 deep eye-pieces. 



The proboscis of the 

 Blow-fly, shown in Fig. 99 

 a favourite preparation 

 of the late Mr. Topping 

 is an excellent object for 

 testing the defining power 

 of an objective of low 

 amplification ; the outlines 



may be fairly sharp, yet the details of the pseudo-tracheae 

 will not be clear, well defined, and free from colour under 



