192 Extracts from Mr. H. E. Fripps Translation of 
thumb " is avoided. The optical constants of each piece of glass 
are obtained from trial-prisms by means of the spectrometer. 
Each constituent lens is ground as nearly as possible to its pre- 
scribed dimensions and accurately fitted. In the highest-power 
objectives only is the lens distance left variable, in order that slight 
deviations from accuracy may be adjusted. And thus it has been 
shown beyond dispute that a well-grounded theory, combined with 
rational technic al processes, may be successfully substituted for 
empirical practice in the construction of the microscope. 
The fact that an amount of angular aperture, which is un- 
known in any other instrument, comes here into question, renders 
the accepted ideas of " aberration " entirely useless, and the result 
of investigations which were undertaken in order to bring the 
question to some issue, was the discovery that an important feature 
in the optical functions of the microscope had been hitherto over- 
looked. In all previous explanations or interpretations it has been 
accepted as a self-understood proposition that the formation of an 
image of an object in the microscope takes place in every particular, 
according to the same dioptric laws by which images are formed in 
the telescope, or in the camera ; and it was, therefore, tacitly pre- 
mised that every function of the microscope was determined by 
the geometrically traceable relations of the refracted rays of light. 
A rigorous examination of the experiences upon which the tradi- 
tional distinction of " defining " and "resolving" powers is founded, 
has shown that the proposition is not admissible. It holds good, 
indeed, for certain cases, capable of definite verification, but for the 
generality of objects, and particularly for those objects on which 
the microscope is supposed to exhibit its highest quality of per- 
formance, it appears that the production of microscopic images is 
closely connected with a peculiar and hitherto neglected physical 
process, which has its seat in and depends on the nature of the 
ohjed itself, although the measure of its effect stands in direct 
dependence upon the construction of the objeetive. It is hence 
possible not only to fix the limits of the visible, beyond which no 
further resolution of structure could be expected, but also to bring 
to light the fact that a microscopic image which may be entirely 
free from error in itself, and therefore be supposed to represent in 
all cases the true structure of an object, nevertheless does not do so 
for a whole class of objects and observations. 
In addition to those images of the object which are thrown off. 
by the lenses of the microscope, a series of associated images of the 
aperture are simultaneously thrown off, which together form an 
image of the outwardly projected plane of aperture. This latter 
(aporture image) is thus associated with the final virtual image of 
the object, and appears at the eye-point, so called, above the ocular 
