USB OF THE MICROSCOPE. 55 



aperture. It may be known by the sharp outline given 

 to the image of an object, which is not much impaired by 

 the use of stronger eye pieces. 



Resolving power is the capability an instrument has of 

 bringing out the fine details of a structure, and depends 

 mainly on the angle of aperture of the objective, or the 

 angle formed by the focus and the extremities of the 

 diameter of the lens. On this account the increase of the 

 angle of aperture has been a chief aim with practical 

 opticians. 



Penetrating power is the degree of distinctness with 

 which the parts of an object lying a little out of focus 

 may be seen. Objectives which have a large angle of 

 aperture, and in consequence great resolving power, are 

 often defective in penetration, their very perfection only 

 permitting accurate vision of what is actually in focus. 

 Hence for general purposes a moderate degree of angular 

 aperture is desirable. 



Flatness of field of view is also a necessity for accurate 

 observation. Many inferior microscopes hide their im- 

 perfection in this respect by a contracted aperture in the 

 eye-piece, by which, of course, only a part of the rays 

 transmitted by the objective are available. 



Object-glasses whose focal length is greater than half 

 an inch are called low powers. Medium powers range 

 from one-half to one-fifth of an inch focal length, and all 

 objectives less than one-fifth are considered high powers. 



For definition with low power objectives, the pollen 

 grains of hollyhock, or the tongue of a fly, or a specimen 

 of injected animal tissue, will be a sufficient test. The 

 aperture should be enough to give a bright image, and 

 the definition sufficient for a clear image. A section of 

 wood, or of an echinus spine, will test the flatness of the 

 field. 



Medium powers are seldom used with opaque objects 

 unless they are very small, but are most useful* with 



