PART III. 

 TESTING THE MICROSCOPE. 



I. 

 TESTING THE OPTICAL POWEE IN GENEEAL. 



As we have shown in a previous chapter that all properties which 

 influence the optical power of a Microscope, and therefore also the 

 sharpness and brightness of its images, are based firstly upon the 

 absence of both kinds of aberration, and secondly upon the mag- 

 nitude of the angle of aperture, we might therefore confine the 

 testing of the instrument to these two points, and to the deter- 

 mination of the magnifying power. In estimating the quality of a 

 Microscope a rigorous examination is never necessary, as the ques- 

 tion simply is whether it compares favourably with other known 

 instruments and satisfies in general the requirements we are 

 justified in demanding. We will now apply ourselves to this 

 practical question, and afterwards follow out the separate elements 

 which enter into consideration on a more special testing. 



1. ABSOLUTE POWER OF DISCRIMINATION. 



Let us inquire, in the first place, how the quality of a Microscope 

 may be estimated, setting aside all peculiarities and faults of 

 construction upon which it depends. This is usually done by 

 means of the so-called test-objects, certain details of which are 

 resolved by the better instruments with a given amplification, 

 while those of inferior quality either do not bring them out at all 



