62 VISION WITH THE COMPOUND MICROSCOPE 



different obliquities at the object which is a certain kind of perspec- 

 tive difference ; but the above and other observations and experiments 

 show that even here there is essential divergence from the conditions 

 of ordinary vision. 



It is thus plain that whenever aperture is effective in delineation 

 the mode in which it becomes so is not by means of the obliquity of 

 the rays to the object ; while it has already been shown that 

 increase of light, always concomitant with the use of immersion 

 objectives, is a relative advantage, but no part of the explanation of 

 the superior action of the combination of lenses. Angle is demon - 

 strably not the true basis for the comparison of objectives ; it fails 

 in regard to aperture in general, so far as it has relation to opening ; 

 it fails equally in regard to the number of rays and the quantity of 

 light admitted to the system of lenses ; while its failure in regard to 

 the delineating power of objectives is everywhere seen and admitted. 



At the same time it is plain that the cause of increased power of 

 performance in the objective is directly connected with the larger 

 opening or ' aperture ' of the immersion and homogeneous systems. 

 In other words, it becomes clear that something is admitted into the 

 objectives with greater apertures which contributes to the formation 

 of an image, such as objectives of lesser aperture cannot form 

 because their ' openings ' or ' apertures ' cannot admit that * some- 

 thing.' 



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 microscopi- 

 cally 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 alterna 

 tions of dark and light occur. 



If a piece of wire be held in a strong beam of divergent light so 

 that its shadow fall upon a white surface, the shadow w r ill not be 

 sharp and black, but surrounded by luminous fringes having the 

 colours of the spectrum, and the centre, where the black shadow of 

 the wire should be, is a luminous line, as if the wire were transparent. 

 This phenomenon, as is generally known, is due to the inflection of 

 the diverging rays on either side of the wire. The inflected rays, in 

 passing over one edge of the wire, meet the rays inflected by the 

 other edge and * interfere,' producing alternate increase and diminu- 

 tion of amplitude of oscillation or undulatory intensity, and giving 

 rise to coloured fringes if white light is used, and if homogeneous 

 light be employed giving origin to alternate bands of light and dark, 

 the centre always being luminous. 



Again, if a disc perforated with a very small hole in the centre 

 be held in a pencil of diverging light, those undulations which pass 



