292 SUMMARY OF CURRENT RESEARCHES RELATING TO 



appears to have a tendency towards an asymptote parallel to the 

 base line. 



A condensed and summarized statement is given of the theoretical 

 principles on which the compound Microscope is based, including the 

 author's now well-known views on the formation of the images of 

 minute objects in microscopical vision, together with observations on 

 the important function of aperture in the Microscope, and the increase 

 of aperture obtained by immersion lenses as compared with dry. 



The remainder of the paper is devoted to a consideration of the 

 possible ways and means by which, in the future, new successes may 

 be hoped for, " the most important practical advantage of a rational 

 theory of the Microscope being that, destroying mere vague hopes, it 

 enables a proper direction to be given to the aims of the inventor." 



With regard to a still further extension of aperture beyond 1 • 5 

 (the refractive index of crown glass), the author suggests that it may 

 be thought that in process of time transparent substances, available for 

 the construction of objectives, will be discovered, whose refractive 

 index will far exceed that of our existing kinds of glass, together with 

 immersion fluids of similarly high refractive power, so as to give new 

 scope to the immersion principle. What, however, he asks, will be 

 gained by all this ? We shall perhaps, with certain objects, such as 

 diatoms, discover further indications of structure where we now see 

 bare surfaces; in other objects, which now show only the typical 

 striations, we shall see something more of the details of the actual 

 structure by means of more strongly diffracted rays ; but we should 

 get on the whole little deeper insight into the real nature and com- 

 position of the minuter natural forms, even should the resolving power 

 of the Microscope be increased to twice its present amount ; for, what- 

 ever part of the structure cannot at present be correctly represented 

 on account of its small size, will then also give an imperfect image, 

 although presenting a somewhat higher degree of similarity than 

 before. If, therefore, we are not to rest upon conjectures which 

 surpass the horizon of our present knowledge (as, for instance, would be 

 the expectation of the discovery of substances of considerably higher 

 refractive power than has hitherto been found in any transparent sub- 

 stance), our progress in this direction in the future will be small, and 

 the domain of microscopy will only be very slightly enlarged, the more 

 so because every such advance, however great, will be but of limited 

 utility to science, on account of very inconvenient conditions. For a 

 given extension of the aperture can only render possible a corre- 

 spondingly enhanced performance of the Microscope when the object is 

 surrounded by a medium whose refractive index at least equals that 

 aperture. If the Microscopes of the future should utilize the high re- 

 fractive power of the diamond, all the objects would have to be im- 

 bedded in diamond, without any intervening substance. The result of 

 this consideration is, therefore, that as long as aperture serves that 

 specific function, which experiment and theory compel us to ascribe to it 

 at present, there is a limit to the further improvement of the Microscope, 

 which, according to the present condition of our knowledge, must be 

 considered as insurmountable. The optics of the day have already so 



