THE MICROSCOPE 



central and peripheral rays focussing at different 

 points. 



(b) Correction of Chromatic Aberration. Chromatic 

 aberration gives rise to a coloured fringe around the 

 edges of objects due to the fact that the different- 

 coloured rays of the spectrum possess varying refran- 

 gibilities and that a simple lens acts toward them as a 

 prism. 



(c) Flatness of Field. The ideal visual field would be 

 large and, above all, flat; in other words, objects at the 



periphery of the field would be as 

 distinctly " in focus " as those in the 

 centre. Unfortunately, however, 

 this is an optical impossibility and 

 the field is always spherical in shape. 

 Some makers succeed in giving a 

 larger central area that .is in focus 

 at one time than others, and al- 

 though this may theoretically cause 

 an infinitesimal sacrifice of other 

 qualities, it should always be 

 sought for. Successive zones and 

 the entire peripheral ring should 

 come into focus with the alteration 

 of the fine adjustment. This simultaneous sharpness 

 of the entire circle is an indication of the perfect cen- 

 tering of the whole of the lenses in the objective. 



(d) Good Definition. Actual magnification is, within 

 limits, of course, of less value than clear definition and 

 high resolving power, for it is upon these properties 

 we depend for our knowledge of the detailed structure 

 of the objects examined. 



(e) Numerical Aperture (N. A.}. The numerical 

 aperture may be defined, in general terms, as the ratio 

 of the effective diameter of the back lens of the objective 

 to its equivalent focal length. The determination of 

 this point is a process requiring considerable technical 



FIG. 48. Huyghenian 

 eyepiece. 



