CORRESPONDENCE. 
Proressors HELMHOLTZ AND ABBE ON THE OPTICAL 
Powers oF THE Microscope. 
To the Editor of the ‘ Monthly Microscopical Jowrnal.’ 
Currron, December 20, 1876. 
Sir,—The writings of Professor Abbe, of Jena, on various subjects 
connected with optical instruments,* are little or scarcely at all 
known in England. But a translation of his essay on the optical 
capacity of the microscope appeared, in 1875, in vol. i. N.S. of the 
‘ Proceedings of the Bristol Naturalists’ Society’; extracts from which 
were also published in the ‘Monthly Microscopical Journal’ of the 
same year. An essay of Professor Helmholtz, on the optical capacity 
of the microscope, also appeared in the Bristol Naturalists’ Society’s 
‘ Proceedings’; and subsequently in the ‘ Monthly Microscopical 
Journal,’ in 1876. 
In the new edition of Niageli and Schwendener’s excellent mono- 
graph on the theory of the microscope and its practical application, 
Professor Abbe’s investigations are mentioned in various chapters as 
introducing a new phase in the solution of many difficult questions 
connected with the optics of this instrument and with the theory of 
the limits of the visible. The researches of Professor Abbe are 
therein estimated as an important contribution, and a decided step in 
advance. 
Amongst many instructive discussions I may refer to a particular 
result obtained by Professor Abbe, which is dwelt upon with much 
force in his essay,f namely, that a large class of objects when placed 
under the microscope and illuminated in the usual way not only 
transmit ordinary pencils which follow their regular and unaltered 
course, but also in virtue of some speciality of internal structure 
diffract other rays, so that a mixture of ordinary and diffracted rays 
is transmitted to the objective. If then the objective have sufficient 
aperture to take in some of the diffracted as well as the ordinary rays, 
two sets of images are produced. The general outlines and larger 
detail of the object are geometrically delineated by the ordinary rays, 
while images of the finer details which caused ditiraction of the illu- 
minating rays as they passed through the object, are reproduced by the 
reunion of the diffracted rays on the same focal plane as that in 
which the geometrical image is formed. And consequently the per- 
fection of the complete image would depend in such cases upon large 
aperture and perfect focussing function of the system of lenses. If 
* For example. His long article in the ‘Jenaische Zeitschrift,’ vol. viii. 
1874, on “New Apparatus for Determining the Refracting and Dispersing 
Powers of Solids and Fluids,” in which the mathematical principles are discussed 
on which his refractometer and spectrometer are constructed. 
t+ See ‘Archives fiir Mikroscop. Anatomie,’ vol. ix. 1874; ‘ Beitrige zur 
Theorie des Mikroscopes;’ or Bristol Naturalists’ Society’s ‘ Proceedings,’ vol. i. 
N.S. 1875. 
