The Structure of Coloured Bodies. 207 



•of shortest wave-length, namely, violet or blue, so do small 

 particles always first reflect light of the same colours, 

 thus producing the phenomenon of opalescence. The 

 blue of the sky, of smoke, and of steam is of this 

 nature. That such transparent particles before attaining a 

 certain size are invisible, is well exemplified in the case of a 

 jet of steam, in which, in immediate proximity to the nozzle 

 before the particles have run together by condensation and 

 thus augmented in size, they are invisible, but assume a blue 

 colour so soon as the diameter of the particles is equal to a 

 quarter wave-length of this colour. Again, in the case of a 

 structure composed of fine lines. Abbe has conclusively 

 demonstrated that the microscopic image of such structure 

 is constituted by the superposition of the ordinary or 

 dioptric image and the interference images formed by 

 diffraction, and that when the diffraction images are 

 ■obstructed by diminishing the aperture of the objective, 

 or otherwise, the appearance of such object may be modified 

 so as to present the most varied appearances, or to present 

 an absolute blank, according as the diffraction images are 

 partially, or wholly, excluded from taking part in the image 

 formation. Of the truth of these facts any one can easily 

 satisfy himself, since Messrs. Zeiss and Son, the opticians of 

 Jena, supply apparatus of the most simple kind, by means 

 of which the part played by diffraction in image formation 

 is rendered apparent. This variation in the appearance of 

 such objects where examined by the microscope has called 

 forth the opinion expressed in a recent publication, " The 

 Microscope in theory and pratice," Naegeli and Schwenderer, 

 p. 235, that "under these circumstances every attempt 

 to discover the structure of finely organised objects, as, for 

 instance, diatom valves, by the mere observation of their 

 microscopic images, must be characterised, is wholly 

 mistaken." 



Seeing now that the microscopic resolution of structures, 



