On Lnprovements of the Microscope. By Prof. E. Abbe. 29 



type, it would be necessary to use eye-pieces of inconveniently short focal 

 length to secure the necessary magnifying power. In the case of these 

 weaker objectives, therefore, those of sliorter focal length should be 

 retained than are in themselves required for a given aperture. 



A set of apochromatic objectives is constructed in the workshops of 

 Zeiss in accordance with the principles here explained. In this series 

 the apertures rise from • 3 to 1 • 4 ; in each class — dry, water-immersion, 

 and homogeneous-immersion — the theoretical maximum of aperture is 

 realized to within 7 per cent, or less. 



How far these innovations will prove of advantage in the scientific 

 use of the Microscope can only be determined by long practice. As 

 regards the point which is of the first importance for the extension of 

 microscopic perception, the aperture, no essential change is introduced 

 into practical optics by the new materials. Although a slight increase 

 in the aperture is obtained, at least in comparison with that which has 

 hitherto been advantageously employed with objectives for systematic 

 work, yet this advantage is comparatively too small to be referred to as 

 of decided importance. A substantial practical gain can only be expected 

 from the internal perfection of the construction in the matter of the 

 collection of the rays. Everything, therefore, is reduced to the question 

 how far the visible action of this perfected construction which was 

 indicated at the outset can be practically utilized in the Microscope, 

 and herein are included the more complete utilization of aperture, or 

 the reduction of the difference which has hitherto always existed between 

 the theoretical and practical performance ; the greater precision and 

 distinctness of perception which is without doubt secured by the more 

 complete collection of the rays ; and, finally, the essentially more 

 favourable conditions which are introduced in the operations of photo- 

 micrography. 



"Whatever may prove to be the final result, the principle of con- 

 struction of objectives as here developed must in any case lay claim to 

 a certain interest from the purely optical point of view, as regards the 

 essentially higher order of collection of rays which is realized by it. 



In the language of dioptrics this order is determined by the number 

 of rays different in direction of incidence or in refrangibility, which, 

 in virtue of the conditions fulfilled by the optical system, are completely 

 united in a single point in the axis. 



Upon the number of these rays depend the greater or less limits 

 within which the other rays may vary which are not completely, but 

 only approximately united ; and these limits are a natural measure of 

 the more or less complete concentration of the rays altogether. 



In this sense an ordinary simple glass lens represents a collection 

 of the rays in its axis of the first order. The objectives of the large 

 telescopes which are made now, exhibit for the most part a concentration 

 of the third order, and it is only those telescopes which are made 

 strictly after the Fraunhofer or the Gauss type of construction which 

 attain the fourth order. 



Now of the Microscope objectives which are here considered, those 

 with the greatest aperture in the different classes have a concentration 



