116 MICROSCOPIC OBJECTS. 



The flutings of the shells of some of the Bacillanaii 

 Infusoria, as the Navicula Hippocampus,"^ are among the 

 most difficult test- objects discovered ; they require a 

 power of 400 diameters to bring them out distinctly. 



• " The defining power of microscopes and engiscopes de- 

 pends on their capability of collecting together all the rays 

 from any one point of an object — or, in other words, their 

 freedom from aberration — and is independent of their 

 penetration ; for, if we take an engiscope, and view a lined 

 object with the aperture of the objective as it is usually sold, 

 its defining power may be very fair ; but if we enlarge 

 the aperture so as to enable us to develop the lines — 

 which it win then accomplish — ^the defining power of the 

 instrument will be injured to such an extent as to render 

 the outline confused. The great desideratum, then, in 

 microscopes and engiscopes, is to obtain these two quahties 

 combined ; which, however, is only rarely attained. 



' ' Cylindrical or spherical bodies appear the best suited 

 for ascertaining the goodness of an instrument as regards 

 definition; and the following examples, which are pre- 

 faced by remarks on the method of illuminating them, I 

 deem sufficient for this end. 



"In the preceding class of objects, oblique diverging 

 mys appear to be essential for the development of their 

 structure ; the degree of obhquity varying, however, with 

 different fined specimens. The extremes of this variation 

 are the Podura plumbea and Pieris Brassica, the delicacy 

 * History of Infusoria, page 207, fig. 145. 



