ARRANGEMENT OF APPARATUS. 103 



careful to employ suitable objectives with certain 

 objects. Low powers should not possess excessive 

 angular aperture, as their utility depends more upon 

 their penetration (that is their power of showing 

 with sharp definition different parts at different 

 depths) than it does on their resolution (or capa- 

 bility of differentiating very fine peculiarities of 

 structure lying on the same plane). On the other 

 hand, high powers are not required to show the re- 

 lation existing between the different parts of a struc- 

 ture lying at different depths, but to define the 

 delicate markings on the surface of those, whose 

 relations at different depths have already been in- 

 vestigated by lower powers. What has before been 

 said about the relation existing between NA and 

 penetration, will show the folly of expecting a lens 

 to combine, great penetration, magnifying power, 

 and angular aperture, at the same time. 



Lately M. Tisandier has described three methods 

 adopted in Paris. All have at least one great dis- 

 advantage ; they require the employment of electri- 

 city in its most expensive form, this places them 

 beyond the reach of ordinary individuals. 



In the first the virtual image is transformed into 

 a real one by displacing the eyepiece, so that the 

 image, formed by the objective, falls on the opposite 

 side of the ocular; this may < answer with the one 

 inch and lower powers, but with higher powers the 

 light is so much diminished by the interposition of 

 four surfaces of glass between the objective and 

 image, that focussing becomes impossible. 



