182 



THE AMERICAN MONTHLY 



[October, 



and optically void interspaces prove 

 the non-existence of anything which 

 he does not see. 



As much time is often lost by fre- 

 quent repetition of those competitive 

 examinations (which after all lead to 

 no better results than that the obser- 

 ver finds or fancies that one lens 

 performs in his hands more or less 

 satisfactorily than some other lens), 

 it seems worth while to invite atten- 

 tion 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 work- 

 ing capacity of his instrument, mea- 

 suring this by a standard of strict 

 optical requirements. The advantage 

 of substituting some such proceeding 

 for the comparative trial of lens 

 against lens, so long in vogue, can 

 scarcely be disputed. For although 

 the best warrant of a well-constructed 

 lens is the fair reputation of its maker, 

 and the choice of an objective resolves 

 itself for the most part into the selec- 

 tion of the particular make of one or 

 other of the best accredited opticians, 

 still, when the instrument is purchas- 

 ed, its possessor frequently becomes 

 haunted by the desire to pit its per- 

 formance against that of some neigh- 

 bor's instrument, or to match the 

 performances traditionally accepted 

 in our handbooks. 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 their 

 instruments than using them, but also 

 yields to the genuine worker the 

 satisfying conviction that his labor is 

 not frustrated by faulty construcUon 

 and performance of his instrument. 

 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 no other object in 

 view than to acquire such insight into 

 the optical conditions of good per- 

 formance as will enable him to make 

 the best use of his instrument, and 



acquire confidence in his interpreta- 

 tion of what he sees, as well as 

 manipulative skill in examining micro- 

 scopical objects. To the constructor 

 and expert of optical science are left 

 the severer investigations of optical 

 effects and causes, the difficulties of 

 technical construction, the invention 

 of new lens-combinations, and the 

 numerous methods of testing their 

 labors by delicate and exhaustive pro- 

 cesses which require special aptitude, 

 and lie entirely outside the sphere of 

 the microscopist's usual work. 



The mode of testing the optical 

 power of an objective here described, 

 is that devised by Prof. Abbe, and ex- 

 plained in his " Beitrage zur Theorie 

 des Mikroskops." 



The process is based on the follow- 

 ing principle : — 



In any combination of lenses of 

 which an objective is composed, the 

 geometrical delineations of the image 

 of any object will be more or less 

 complete and accurate, according as 

 pencils of light coming from the ob- 

 ject are more or less perfectly focussed 

 on the conjugate focal plane of the 

 objective. On this depend fine defi- 

 nition and exact distribution of light 

 and shade. The accuracy of this fo- 

 cussing function will be best ascer- 

 tained by analysing the course of 

 isolated pencils directed upon differ- 

 ent parts, or zones, of the aperture, 

 and observing the union of the several 

 images in the focal plane. For this 

 purpose it is necessary to bring under 

 view the collective action of each part 

 of the aperture, central or peripheral, 

 while at the same time the image, 

 which each part singly and separately 

 forms, must be distinguishable and 

 capable of comparison with the other 

 images. 



I. The Illumination must there- 

 fore be so regulated that each zone of 

 the aperture shall be rejDresented by 

 an image formed in the upper focal 

 plane of the objective (i. e. close be- 

 hind or above its back lens), so that 

 only one narrow track of light be al- 

 lowed to pass for each zone, the tracts 



