222 ON THE THEORY OF THE MICKOSCOPE. 



theoretical points of view here indicated are quite applicable to 

 an approximative estimate of the performance which may be 

 expected from the instruments of the present day ; and their 

 enunciation may well serve to remove those illusions to which 

 many writers on the microscope seem disposed to give themselves 

 up. Theoretical study of the aberrations of the image-forming 

 rays, and practical experience involving the application of methods 

 to be hereinafter dcsciibed, and the careful testing of a con- 

 siderable number of objectives of recent date from the best work- 

 shops on both sides of the channel, have led Professor Abbe to 

 the conclusion that the numerical value of "necessary amplifica- 

 tion " yet arrived at or attainable, at present, is altogether much 

 lower than might^be^ supposed from the liberal way in which 

 microscopists deal with thousands and tens of thousands. 

 According to his experience, the optical power (capacity) of the 

 most pel feet objectives, the usual forms of illumination being 

 assumed, is exhausted with an eight-fold angular amplification, so 

 that every detail that can be possibly delineated by an objective 

 in its " virtual " image is certainly accessible to any eye possess- 

 ing normal vision, when the tube and ocular, taken together, 

 represent a telescopic magnifying power of eight times. Even 

 this performance is only reached in the case of low and middle 



of the oi)tical poweri of the microscope in any practically realisable way, or 

 one capable of general application ; for apart from the fact that the course 

 of effective rays in this case differs widely from all the formulas which 

 apply in the ordinary modes of using the microscope (at all events, with 

 the higher objectives), it may be demonstrated, as will be seen in the next 

 following chapter, that the resolution of details in such air-bubble images 

 by no means depends upon the dioptric action of the objective alone, but 

 quite as much, or more, upon special and entirely independent influences, 

 extraneous to the action of the microscope. The results of Harting's 

 m.ethod shew, in point of fact, only the limits of a resohdng power which 

 is unconnected with dioptric perfection of the objective, just as in the case 

 of the observations of Koberts' test-plate, or of the Diatoms, though under 

 somewhat altered conditions of illumination. 



J 



