4 ON OPTICAL QUALITY OF MICROSCOPE OBJECTIVES. 



may or ought to be the ultimate hmit of optical performance, or 

 whether any particular lens under examination has actually reached 

 this limit. 



Assuming the manipulation of the instrument and the illumin- 

 ation of the object to be as perfect as possible and, further, that 

 the '^ test object ' ' has been selected with due appreciation of 

 the requirements of perfect optical delineation, a fair comparison 

 can only be drawn between objectives of the same magnifying 

 power and angle of aperture. Which of two or m.ore objectives 

 gives the better image may be readily enough ascertained by such 

 comparison, but the values thus ascertained hold good only for 

 the particular class of objects examined. The best performance 

 realised with a given magnifying power, may possibly exceed 

 expectation, yet still be below what might, and therefore ought 

 to be obtained. On the contrary, extravagant expectations may 

 induce a belief in performances which cannot be realised. The 

 employment of the test objects most in use, is, moreover, 

 calculated to lead to an entirely one-sided estimation of the actual 

 working power of an objective, as for example when '*^ resolving 

 power " is estimated by its extreme limits rather than by its 

 general efficiency : or '' defining power'' by extent of amplification 

 rather than clearness of outline. So that an observer is tempted 

 to affirm that he can discern through his pet lens what no eye 

 can see, or lens show. This happens chiefly with the inexpe- 

 rienced beginner, but not unfrequently, also, with the advocate of 

 extremely high powers in whose mind separation of detail means 

 analysis of structure and optically void interspaces prove the 

 non-existence of any thing which he does not see. 



As much time is often lost by frequent repetition of these 

 "competitive examinations" (which after all lead to no better 

 result than that the observer finds or fancies that one lens 

 performs in his hands more or less satisfactorily than some other 

 lens,) it seems worth while to invite attention to a mode of 

 testing which can be readily practised by any person, with a fair 

 certainty of being able to form a really correct estimate of the 



