i II. /] MICROSCOPE AND ACCESSORIES 25 



Of course when equivalent focus and numerical aperture both differ the 

 problem becomes more complex. 



While all niicroscopists are agreed that the fineness of detail which can 

 be seen depends directly on the numerical aperture of the objective used, the 

 general theory of microscopic vision has two interpretations : 



(A) That it is as with the unaided eye, the telescope and the photo- 

 graphic camera. This is the original view and the one which many are 

 favoring at the present day (see Mercer, Proceedings of the Amer. Micr. Soc. 

 1896, pp. 321-396 ; Wright, Gordon and Beck). 



(B) The other view originated with Professor Abbe, and in the words of 

 Carpenter-Dallinger, pp. 62, 43: "What this is becomes explicable by the 

 researches of Abbe. It is demonstrated that microscopic vision is sui generis. 

 There is and can be, no comparison between microscopic and macroscopic 

 vision. The images of minute objects are not delineated microscopically by 

 means of the ordinary laws of refraction ; they are not dioptrical results, but 

 depend entirely on the laws of diffraction. These come within the scope of 

 and demonstrate the undulatory theory of light, and involve a characteristic 

 change which material particles or fine structural details, in proportion to 

 their minuteness, effect in transmitted rays of light. The change consists 

 generally in the breaking up of an incident ray into a group of rays with 

 large angular dispersion within the range of which periodic alternations of 

 dark and light occur." 



For a consideration of the aperture question, its history and significance, 

 see J. D. Cox, Proc. Amer. Micr. Soc., 1884, pp. 5-39; Jour. Roy. Micr. Soc., 

 i88r, pp. 303, 348, 365, 388; 1882, pp. 300, 4bo; 1883, p. 790; 1884, p. 20; 

 1896, p. 681 ; 1897, p. 71 ; 1898, pp. 354, 362, 592; Mercer, Proceedings Amer. 

 Micr. Soc., 1896, pp. 321-396; Lewis Wright, Philos. Mag. , June, 1898, pp. 

 480-503 ; Carpenter-Dallinger, Chapter II ; Nelson, Jour. Quekett Micr. Club, 

 VI, pp. 14-38 ; Jour. Roy. Micr. Soc., 1906, pp. 521-531 ; A. E. Wright's Prin- 

 ciples of Microscopy ; Conrad Beck, Theory of the Microscope. Gordon, 

 Jour. Roy. Micr. Soc., 1902. 



THE OCULAR 



$ 41. A Microscopic Ocular or Eye-Piece consists of one or more con- 

 verging lenses or lens systems, the combined action of which is, like that of a 

 simple microscope, to magnify the real image formed by the objective. 



Depending upon the relation and action of the different lenses form- 

 ing oculars, they are divided into two great groups, negative and positive. 



$ 42. Negative Oculars are those in which the real, inverted image is 

 formed within the ocular, the lower or field-lens serving to collect the image- 

 forming rays somewhat, so that the real image is smaller than as if the field- 

 lens were absent (Fig. 26). As the field-lens of the ocular aids in the forma- 

 tion of the real image it is considered by some to form a part of the objective 

 rather than of the ocular. The upper or eye-lens of the ocular magnifies the 

 real image. 



