240 0]S- THE THEOEY OF THE MICttOSCOPE. 



of image formation is actually brought about. It can be shewn 

 that the images of the illuminating surface, which appear in the 

 upper focal plane of the objective, (the direct image and the 

 diffraction images) must each represent, at the point of corres- 

 pondence, equal oscillation phases when each single color is 

 examined separately. 



These aperture images, therefore, stand to each other in the 

 same relation as the two mirror images of a flume in Fresnel's 

 experiments on the interference of light. The meeting of the rays 

 proceeding from them must occasion, in consequence of the 

 occurring interference, a periodic alteration of light and dark, 

 whose relative form and dimensions depend on the number, 

 disposition, and mutual distance from each other of the interfering 

 illuminated surfaces. The delineation of structure seen in the field 

 of the microscope is in all its characters, — those which are conform- 

 able with the real constitution of the ohject as well as those which are 

 not so — nothing more than the result of this jyrocess of interference 

 occurring ivhere all the image-forming rays encounter each other. 

 The relation existing between the linear distances from the axis 

 of the microscope uf constituent elements of the aperture image, 

 and the various inclination of rays entering the objective, 

 (explained and formuUirised in section II., 4) taken together with 

 the dioptric analysis of the microscope, as set forth in 6, afford all 

 the data necessary for complete demonstration of the above 

 positions. From them may be deduced that in an achromatic 

 objective the interference images, for all colors, coincide, and 

 yield as a total effect achromatism, thus differing from all other 

 known interference phenomena. Fuither, that the proportionate 

 dimensions of the images so produced always depend in such wise 

 upon those of the actual structure as the linear magnifying power 

 of the microscope would bring, according to the dioptric law of 

 formation of images, whatever be the arrangement of the optical 

 parts or the mode of illumination. And all the facts stated in 

 the 16th paragraph are not only fully accounted for, but beyond 

 this it is possible to calculate beforehand, in all its details, the 



