02 ARRANGEMENT OF APPARATUS, ETC. 



results as the eighth and one twenty-fifth of Messrs. 

 Powell and Leland. This we think was owing to 

 the proper relaticn between the size of the object 

 and the aperture of the glass being strictly observed; 

 the adjustment for cover glass being made at the 

 same time with extreme accuracy; and I think prin- 

 cipally because the balance between the power and 

 aperture of the condenser, and that of the object, 

 was more easily equalised with these objectives than 

 the others. 



Good results can never be obtained unless we are 

 careful to employ suitable objectives with certain 

 objects. Low powers should not possess excessive 

 angular aperture, as their utility depends more upon 

 thsir 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- 



