0|)tial quality 0f ^\kx0m\n (^hluim^. 



BY H. E. FRIPP, M.D. 



IN ordinary practice, microscope objectives, if tested at all by 

 their possessors, are simply subjected to a comparison of 

 performance with other lenses tried upon the same '^ test objects." 

 The relative excellence of the image seen through each lens may, 

 however, depend in a great part upon fortunate illumination, and 

 not a little upon the experience and manipulative skill of the 

 observer 3 besides which any trustworthy estimate of the perform- 

 ance of the lens under examination involves the consideration of a 

 suitable test object, as well as the magnifying power, and angle 

 of aperture of the objective. The structure of the test object 

 should be well known, and the value of its ''markings" if 

 intended to indicate microscopic dimensions, should be accurately 

 ascertained, care being taken that the minuteness of dimensions 

 and general delicacy and perfection of the test object be adapted 

 to the power of the lens. A fairly correct estimate of the relative 

 performance of lenses of moderate magnifying power may 

 doubtless be thus made by a competent observer, but it is not 

 possible from any comparisons of this kind to determine what 



