OW THE THEOKY OF THE MICEOSCOPE. 213 



infinitely fine. On the other hand there must be taken 

 into account the peculiar circumstance of altering divergence 

 of pencils of large angular aperture after entering the 

 objective; and that equally peculiar condition which obtains 

 in the ocular of expanding superficies of image due to widely 

 diverging rays. But it may be shown that the production of a 

 fairly perfect picture, under the conditions of aperture angles and 

 image-forming angles above described, cannot be effected in any 

 optical apparatus otherwise than by such a distribution of specific 

 focussing function, and specific act of amplification over different 

 parts of the instrument; and, consequently, that the admitted 

 advantages of the compound microscope arise from this combin- 

 ation of the several functions of ** objective" and ''ocular." It 

 follows, however, from this, (so long at least as present 

 principles of construction are applied) that the actual boundary- 

 line between ''objective" and "ocular" function is not to be 

 sought where the image produced by the "objective" is presented 

 to the "ocular," (at the field lens) but rather in the "objective" 

 itself, where the rays which entered in a divergent direction are 

 rendered, by repeated refractions, parallel, from which line 

 (plane) they become by further refractions convergent on their 

 course towards the ocular. 



YI. Consistently with this result an analytical diagram of a 

 more distinctive kind must be substituted for that in 

 ordinary use whenever it is required to ascertain the quality of a 

 microscopic image, by reference to the conditions which are really 

 determinative, and which, moreover, may be advantageously 

 applied as a basis for determining the quantitative relations of the 

 optical action. According to this analysis the first step or act in 

 the image-forming process consists, not in the production of a 

 reversed image by the objective in front of, or within, the 

 ocular, but rather in the production of a "virtual" image at 

 endless distance with parallel rays (such as is seen when an object 

 situate in the principal focus of any simple converging lens is 

 observed by the eye placed behind it). The second act comprises 



