PROFESSOR ABBE’S METHOD OF TESTING OBJECTIVES. 107 
PROFESSOR ABBE’S METHOD OF TESTING 
OBJECTIVES. 
Extracted from the Journal of the Royal Microscopical Society. 
'HE late Dr. H. E. Fripp published, in 1877,* an account of 
Prof. Abbe’s method of testing the optical quality of objectives, 
which he suggested might be usefully transferred to the pages of 
this journal. Various causes have hitherto prevented this, but we 
are now able to print it :— 
In ordinary practice, microscope objectives, if tested at all by 
their possessors, are simply subjected to a comparison of per- 
formance with other lenses tried upon the same “test objects.” 
The relative excellence 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 manipulative skill of the 
observer ; besides which any trustworthy estimate of the perform- 
ance of the lens under examination involves the consideration of 
a suitable test object, as well as the magnifying power and aperture 
of the objective. The structure of the test object should be well 
known, and the value of its ‘‘ markings,” if intended to indicate 
microscopical dimensions, 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 doubt- 
less 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 of the instrument and the illumination 
of the object to be as perfect as possible, and, further, that the 
test-object has been selected with due appreciation of the require- 
ments of perfect optical delineation, a fair comparison can only be 
drawn between objectives of the same magnifying power and aper- 
ture. Which of two or more 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 realized witha given magnifying 
power may possibly exceed expectation, yet still be below what 
might, and, therefore, ought to be obtained. On the other hand, 
* Proc. Bristol Naturalists’ Soc., ii. (1876-9) pp. 3-11 (2 figs.). 
