TESTING OBJECTIVES 383 



The structure of the test-object should be well known, find the 

 value of its ' markings ' if intended to indicate microscopical dimen- 

 sions should be accurately ascertained, care being taken that the 

 minuteness of dimensions and general delicacy and perfection of the 

 test-object should 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 may or ought to be the ultimate limit of optical 

 performance, or whether any particular lens under examination has 

 actually reached this limit. 



Assuming the manipulation ^the instrument and the illumina- 

 tion 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 be- 

 tween objectives of the same magnifying power and aperture. Which 

 of two or more objectives gives the better image may be readily 

 enough ascertained by such comparison, but the values thus ascer- 

 tained 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 other hand, 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 ' denning 

 power ' by extent of amplification rather than by 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 inexperienced beginner, but not unfrequently also with 

 the more experienced worker who advocates the use of great amplifi- 

 cation, in whose mind separation of detail means analysis of struc- 

 ture, 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 com- 

 petitive 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 consider the value of a mode of testing which can be readily 

 applied whatever its value may be. A short and easy method of 

 testing an objective not by comparison with others only, but by 

 itself and on its own merits affords not only the most direct and 

 positive evidence of its qualities to those who are more concerned 

 in proving these instruments than using them, but also yields to 

 the genuine worker the satisfying conviction that his labour is 

 not frustrated by faulty construction and performance of his instru- 

 ment. It is, however, to be borne in mind that the microscopist, in 

 any scrutiny of the quality of his lenses which he may attempt, has 

 110 other object in view than to acquire such insight into the optical 



