IMPORTANCE OF THE ANGLE OF APERTURE. 97 



the power of differentiation or resolution, in accordance with the 

 literal meaning of those terms. On the other hand, however, the 

 facts above mentioned prove just as clearly that the dioptric 

 reunion in one image-surface of the pencils issuing from the object 

 does not explain the delineation of very fine structures, but that 

 considerations external to the Microscope must necessarily be taken 

 into account. 



As already pointed out, it is the diffraction (or an equivalent 

 deviation due to refraction or reflexion) of light in the object 

 which produces the image-forming rays of the fine structural 

 details. These diffracted rays produce well-defined diffraction or 

 interference images in the upper focal plane of the objective 

 where they interfere, and remain in the microscopic image, and 

 therefore take part also in the final virtual image. 



If we shut out these interference images by diaphragms, or if 

 the angular aperture of the objective is too small to admit at 

 least the first pencil of rays produced by diffraction, as well as 

 the un diffracted light, the corresponding details disappear in the 

 microscopic image, a valve of Pleurosigma shows neither squares 

 nor lines, and fine rulings upon glass appear as a homogeneous 

 surface. 



The signification of the angle of aperture consists, therefore, in 

 the fact that it renders possible the production of the interference 

 image, in which alone the finer structures are included. 1 



1 Our former discussions on this point were not exactly based, as Harting 

 asserts ("Mikr." 2nd ed. i. p. 278), upon the theory of illumination, differing 

 somewhat from his own, but entirely upon the supposition, generally recognized 

 up to that time, that the Microscope image is a dioptric one. So far as this is 

 the case, the proposition which was then put forward is still correct, viz., that, 

 disregarding the amount of light and amplification, the Microscope developed 

 no other power than that which progressed at an equal pace with the elimination 

 of the two aberrations : and the same may be said in general also of the 

 conclusions derived therefrom. By the demonstration, however, that the 

 phenomena of diffraction play so important a part in the formation of the 

 image, the case has changed essentially in this as in so many allied questions, 

 and it is useless here to repeat the views which, according to the new explana- 

 tion of the matter, could have only a very secondary practical importance. 



On the other hand, it may not be superfluous here to lay stress upon the 

 fact that hitherto none of the treatises on the Microscope known to us have 

 scientifically established, even as a fact, the effect produced by the angle of 

 aperture, much less explained it. 



