ON THE THEOKY OF THE MICROSCOPE. 231 



and this is irrespective of the circumstance that the conetitution 

 of the object admits or entirely excludes the possibility of shade 

 eflfects. 



(v.) A structure of the supposed kind, which is not revealed 

 by an objective used with direct illumination will not be 

 rendered Wsible by inclining the olject itself at any angle to the 

 axis of the microscope, even when, lying at right angles with the 

 axis, it is perfectly resolved by oblique illuminations. Resolu- 

 tion, however, follows at once when the incident light is directed 

 perpendicularly to the plane of the object, as it lies inclined to the 

 axis. Hence the increased effect of obliq[ue illumination depends 

 solely on the inclination of the rays towards the axis of th% 

 instrument, and not upon the oblique incidence of light on the 

 object.* 



The facts here brought forward shew, on the one hand, the 

 reality of a special optical quality, directly related with angular 

 aperture of the objective, yet independent of any special perfection 

 or amplifjing power possessed by it, and shew it to be a 

 ''resolving " power or capacity of separating minute detail, con- 

 formably with the literal sense of the term employed. On the 

 other hand, they shew unequivocally that the delineation of 

 images of minute details of structure must take place under con- 

 ditions essentially different from those under which the contour 

 outlines of larger parts are formed. In all cases where a 

 '* resolving " power of this kind — that is to say, a direct influence 

 of angular aperture, whether positive or negative — is in operation, 

 the dioptric reunion of rays proceeding from the several points of 

 the object in the focal plane of the image is most certainly not to 

 be accounted an adequate explanation of the images of such details 

 of an object, for on such a supposition the differences experi- 

 mentally ascertained and above described would remain absolutely 



* Vide, "Wenliain ia monthly Microscopic Journal, April 1, "On a method 

 of obtaining obKque "vision of surface of structure, <S:c." The optical 

 principle enunciated by ^Ir, Wenham is totally irreconciliable with Prof. 

 Abbe's theory and experimental iQTestigations. 



