108 THE MICROSCOPICAL NEWS. 
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 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 advocate of extremely high powers, in whose mind 
separation of detail means analysis of structure, and optically void 
interspaces prove the non-existence of anything 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 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 working capacity of his 
instrument, measuring this by a standard of strict optical require- 
ments. 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 selection 
of the particular make of one or other of the best accredited 
opticians, still, when the instrument is purchased, its possessor 
frequently becomes haunted by the desire to pit its performance 
against that of some neighbour’s instrument, or to match the per- 
formances 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 
labour is not frustrated by faulty construction 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 performance as will 
enable him to make the best use of his instrument, and acquire 
confidence in his interpretation of what he sees, as well as mani- 
pulative skill in examining microscopical objects. To the con- 
structor and expert of optical science are left the severer investi- 
