THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL 



Vol. IV. 



Boston, October, 1883. 



No. 10. 



Abbe's Method of Testing Ob- 

 jectives. 



[The following article, by the late 

 Dr. H. E. Fripp, was published in the 

 Proceedings of the Bristol Naturalists' 

 Society, in the year 1877, and has 

 been reprinted in the Journal of the 

 Royal Microscopical Society. It is 

 of such general interest that we copy 

 it from the last mentioned _/^«r;zrt;/, in 

 full. — Ed.] In ordinary practice, 

 microscope objectives, if tested at all 

 by their possessors, are simply sub- 

 jected to a comparison of performance 

 with other lenses tried upon the same 

 " test objects. " The relative excel- 

 lence 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 

 manipulativ^e skill of the observer; 

 besides which any trustworthy esti- 

 mate of the performance of the lens 

 under examination, involves the con- 

 sideration of a suitable test object, as 

 well as the magnifying power and 

 aperture of the objective. The struc- 

 ture of the test object should be well 

 known, and the value of its " mark- 

 ings, " if intended to indicate micro- 

 scopical dimensions, should be accu- 

 rately 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 particu- 

 lar lens under examination has 

 actually reached this limit. 



Assuming the manipulation of the 

 instrument and the illumination of the 

 object to be as perfect as possible, 

 and, further, that the test objec: 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 aperture. 

 Which of two or more objectives gives 

 the better image may be readily 

 enough ascertained by such compari- 

 son, but the values thus ascertained 

 hold good only for the particular 

 class of objects examined. The best 

 performance realized 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, extra- 

 vagant expectations may induce a 

 belief in performances which cannot 

 be realized. 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 pow- 

 er " 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 inexpe- 

 rienced beginner, but not unfrequent- 

 ly also with the advocate of extremely 

 high powers, in whose mind separation 

 of detail means analysis of structure, 



