292 ON LIGHT. 



is no diminution of the colours, or alteration of their 

 forms. It is to the interval between the surfaces that we 

 have to look for their origin. 



(75.) Here, then, we have LIGHT brought face to face 

 with SPACE, and no escape! What happens at or be- 

 tween these surfaces'? How is it that while a single 

 surface reflects a dispersed beam of light indifferently 

 over its whole extent, this indifference is destroyed by 

 placing another reflecting surface behind it; and the 

 reflexion (at least the effective reflexion as regards the 

 spectator] rendered impossible when the second surface 

 is at a certain distance, or at certain distances, from the 

 first; while if placed at intermediate distances, it is 

 either not at all affected, or only to a certain extent 

 enfeebled 1 and that, when there is nothing, or at least 

 nothing realizable to any of our methods of observation, 

 between them? This is the problem before us, reduced 

 to its simplest terms, a problem which the corpuscular 

 theory of light resolves imperfectly and unsatisfactorily, 

 and the undulatory fully and without reserve. 



(76.) When instead of using the prismatic spectrum 

 (of which it is next to impossible to insulate from the 

 rest a ray of perfectly definite refrangibility) to illuminate 

 the film, we employ artificial light (such as that of a 

 spirit lamp with a salted wick, which may be considered 

 as almost perfectly homogeneous), the rings are seen 

 with extraordinary sharpness; their central spot and their 

 divisions having the blackness of ink, and absolutely in- 

 numerable ; being traceable with a magnifier when too 

 close to be otherwise distinguishable, apparently without 



