228 ON THE THEORY OF THE MICROSCOPE. 



and an examination of their lateral separation and their differences 

 of level, as well in the middle as in the peripheral zones of the 

 entire field, suffice for an accurate definition of the nature and 

 amount of the several errors of correction, each of them appearing 

 in its own primary form. Therewith we also see that which 

 arises from aberrations properly so called (faults of the focussing 

 function) clearly separated from such imperfections or anomalies, 

 "which spring from mere differences of amplification between un- 

 equally inclined and unequally refracted rays. And, moreover, 

 we can eliminate completely, by a simple manipulation, all 

 influence of the ocular on the quality of the image outside the 

 axis.* 



Assuming the theoretical knowledge and practical experience 

 necessary to carry out such an enquiry properly, and to estimate 

 its results correctly, the mode of procedure above described 

 affords so exhaustive an analysis of the qualities of an objective, 

 that when, in addition, its focal length and angle of aperture are 

 ascertained, its whole capacity of performance may be determined 

 beforehand. For the ordinary requirements of the microscopist, a 

 direct test by means of some natural object will be always 

 preferred; but the occasional application of this method will 

 yield useful data upon which a right judgment may be formed of 

 the expectation which at present may or may not be entertained 

 respecting the quality of the microscope. Whoever has once 

 examined in this manner even good objectives which have proved to 

 be excellent in practice, will be as little disposed to accept 

 childish assertions of their perfectness as to advance on his part 

 absurd pretensions which no one has yet made good. 



* In proof of these statements may be mentioned that by this method, in 

 accordance with theory of dispersion of color, (which is actively going on 

 in every part of the field outside its axis, where objectives of large angular 

 aperture are used) no less than five separate elements are recognized, 

 which, on account of their very unequal practical significance, must be 

 strictly distinguished from each other. 



