410 LIMITS or OPTICAL CAPACITY OF THE MICEOSCOPE. 



large *'apertare" 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 papil. This contracted area 

 — the necessary consequence of the optical combinations used to 

 obtain high amplification — has the same efi'ect as any minute 

 aperture through which aluminous object is viewed, and occasions, 

 as is well known in physics, those diff'ractive eftects which obscure 

 the outlines 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 

 Helmholtz and Abbe on identical physical and physiological basis, 

 is therefore of great importance in its general bearing in physical 

 science, and the precise and comprehensive treatment of it in the 

 following pages worthy of careful study. 



As respects the translation now off'ered, it is only necessary to 

 add that it was undertaken at the same time as that of Prof. 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 countiy, and whose teaching he 

 had himself accepted with pleasure. Ifo mention of English co- 

 temporary work was needed therefore in the brief introductory 

 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. 



