284 SUMMARY OF OUBKENT RESEARCHES RELATING TO 



microscopical optics were brought prominently before microscopists, we 

 were a little surprised to find that principles which it had cost so much 

 time and trouble and money to record should be suddenly trampled upon 

 in what, from our point of view, was a most unreasoning and unreason- 

 able manner. 



As it is possible that the explanation is to be found in the fact that, 

 notwithstanding the shortness of the time, new minds have come upon 

 the scene which were not in being at the time of the old discussions, we 

 propose to consider in detail in this and following numbers of the Journal 

 the various errors above referred to, so that at any rate for the next ten 

 years, wo may hope to be free from similar misapprehensions. 



(1) We will first deal with the notion that the difii'action theory as 

 promulgated by Professor Abbe is affected either in principle or applica- 

 tion by the increase of the theoretical maximum of the apertures of objec- 

 tives from 1'33 (water) to 1'5 (oil). The text on which we found this 

 explanation is a statement quoted in this Journal for 1888, p. 1034, and the 

 full text of which will be found in the place indicated in the footnote,* 



The best answer that can, we think, be given to this notion is the 

 following paragraph from a paper written by Professor Abbe hefore the 

 introduction of homogeneous-immersion lenses, and it will be seen that 

 at that time he assumed the existence of objectives of 1 • 5 and discussed 

 the capabilities of much larger apertures, a point which we need hardly 

 remind our readers, has not yet been reached. 



Professor Abbe said : — " With regard to a still further extension of 

 aperture beyond 1 • 5 (the refractive index of crown glass), 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, 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 ; hut we should get on the ivhole little deeper 

 insight into the real nature and composition of the minuter natural forms, 

 even should the resolving power of the Microscope he increased to twice its 

 present amount ; for, whatever 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 uj)on conjec- 

 tures 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 substance), 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 correspondingly enhanced performance of the Microscope 

 when the object is surrounded by a medium whose refractive index at 



* Eng. Mech., xlviii. (1888) p. 178. 



