156 J. RHEIXBERG ON THE COLLECTED PAPERS OF 



Of high interest also is Treatise IV., "The Optical Means of 

 •i^isting Microscopy." It is a report on the scientific apparatus 

 at the London International Exhibition of 1876. This report 

 is apparently one of Abbe's least-known papers, and yet 

 it contained a summary intelligible to every one of what had 

 been achieved so far, and of future prospects regarding the 

 perfecting of the microscope, the extending of the limits of its 

 capability, and, above all, the new era in the manufacture of 

 glass, in which we find predictions of the results arrived 

 at to-day. The conditions for the production of apochromatic 

 objectives, for the increase in resolving power by the employ- 

 ment of ultra-violet rays, are already here clearly expressed. 

 The remarks on the dependence of the further perfecting of 

 optical systems on the production of new sorts of glass were the 

 spur to the erection of the Jena Glass Works. 



It is scarcely possible to present the optical capabilities of the 

 compound microscope more shortly or pregnantly than is done 

 in a few pages (pp. 135-9) of this treatise. The study of 

 this chapter can be most warmly recommended to every 

 microscopist. 



The investigations on the light intensity in optical instru- 

 ments have perfectly cleared up this difficult subject. Although 

 this has only to do with comparatively simple geometrical and 

 physical matters, the greatest confusion obtained about it in 

 more than one respect; so that Abbe could rightly say elsewhere 

 (p. 69) that since the time of Brewster and Wollaston the theory 

 of illuminating apparatus was the partie honteuse of microscopic 

 doctrine till N'ageli and Schwendener first brought clear and 

 sound conceptions about it. In ;various other places, too, he 

 brings out how the classical exposition of the laws of illumination, 

 which appeared already in the first edition (1865) of Nageli 

 and Schwendener's book on the microscope, formed the starting 

 point of his investigations (cp. pp. 31, 102, 275). 



Only by sharply distinguishing between the geometrical and 

 physical conditions determining the light effect, as is evidenced by 

 contrasting the amount of radiation and the intensity of the 

 radiation — the radiating power — could the confusion be cleared 

 away. The intensity of a source of light depends substantially 

 on the radiating power of its surfaces and on its temperature ; 

 in all effects it acts as a whole. The amount of radiation 



