XJapacity of the Microsco'pe. By Prof. Helmlioltz, 17 
" aperture " of objective by means of diaphragm openings and stops 
than by using stronger light with diminished aperture. Thus the 
management of illumination, and manipulation of the microscope 
to obtain good definition, though for the most part left to empirical 
practice, would be more easily and thoroughly acquired if the 
physiological laws were carefully studied. But another and far 
more serious deterioration of definition arises from excessive diminu- 
tion of area of the image entering the pupil. This contracted area 
— the necessary consequence of the optical combinations used to 
obtain high amplification — has the same effect as any minute 
aperture through which a luminous object is viewed, and occasions, 
as is well known in physics, those diffractive effects which obscure 
the outhnes of an image by making them overlap each other. On 
this fact is founded the whole argument of Professors Helmholtz 
and Abbe respecting the limits of microscopic vision, as well as the 
corollary which directly follows from it respecting the ultimate 
limits of minuteness to be assigned for vision of any and every 
kind of material atoms with the optical apparatus and materials yet 
employed. The theory of the microscope as interpreted by Helm- 
holtz and Abbe on identical physical and physiological bases, is 
therefore of great importance in its general bearing on physical 
science, and the precise and comprehensive treatment of it in the 
following pages worthy of careful study. 
As respects the translation now offered, it is only necessary to 
add that it was undertaken at the same time as that of Professor 
Abbe's essay, and with exactly the same motives. Our readers will, 
it is hoped, bear in mind that the translator's object was simply to 
make known to those who could not otherwise so readily inform 
themselves, the views of scientific men abroad, whose authority on 
these subjects is at all events high in their own country, and whose 
teaching he had himself accepted with pleasure. No mention of 
English cotemporary work was needed therefore in the brief intro- 
ductory notice of Dr. Abbe's article. Since its publication, however, 
the translator has been questioned respecting English contributions 
to the theory of the microscope, and he therefore ventures to add a 
few words on this subject. 
One may be well excused from referring to the meagre optical 
chapters in our handbooks on the microscope, which might perhaps 
suit the ' Boys' Own Book,' but which contain neither demonstra- 
tion nor diagram of the course of rays through any sort of modern 
lens system, nor even a rough application of its very elementary 
statements respecting refraction and reflexion to any special for- 
mulae of constructions, according to which the lens combination of 
an objective would be worked, or by which its performance would 
be tested. Nor can the favourite descriptive chapter of the instru- 
ments of various makers help anyone to a theory of the micro- 
