162 J. RHEIXBERG ON THE COLLECTED PAPERS OF 



Unfortunately the promised second part, on the limits of geometrical 

 optics, has not appeared, though it is to be hoped that parts 

 of it in manuscript may be published later on. 



Treatise X., " On New Methods for improving Spherical 

 Correction, applied to the Construction of Wide-angled Object- 

 glasses," h;is theoretical importance for the calculation of 

 objectives. The nature of the chromatic difference of spherical 

 aberration is here explained theoretically exactly ; and, at the 

 same time, the problem of the illumination of this defect is 

 solved — by an expedient which was of little importance to prac- 

 tical microscopy, certainly — i.e. by employing so-called fluid lenses. 

 But it was by this experimentally shown that, by having a great 

 number of media suitable for the production of lenses at one's 

 disposal, substantial progress could be attained. Only several 

 years later, owing to the introduction of numerous new kinds 

 of glass, and of fluorite, could this result be put to practical 

 use, as is set forth very thoroughly in Treatise XX., " On New 

 Microscopes," and Treatise XXII., " On the Employment of 

 Fluorite for Optical Purposes," which treat of the method of 

 production of apochromats. 



The introduction of homogeneous oil immersions, which Abbe 

 computed at the suggestion of Stephenson, was an epoch of not 

 less importance in the construction of objectives. A quarter 

 of a century before, Amici had already used different oils as 

 immersion fluids, but these could not be designated homogeneous 

 immersion systems. The independence attained of the thickness 

 of the cover-glass, apart from the other great advantages of 

 such objectives, constituted a decided advance. Treatise IX. 

 contains interesting information concerning the origin of these 

 objectives, which have become so extremely important for all 

 delicate work in histology and bacteriology. 



Another series of treatises, which originally appeared in 

 English only, are devoted to lively discussions, principally carried 

 on in English journals of microscopy, concerning the ques- 

 tions of the measurement of aperture, the relation between 

 aperture and magnification, the conditions for stereoscopic 

 vision, all-round vision, illumination by wide-angled cones 

 of light, and the definition of magnified images. In this category 

 are included: Treatise XII., "Some Remarks on the Aperto- 

 meter " ; XV., " On the Conditions of Orthoscopic and 



