PRODUCED BY ELECTRO-CHExMICAL ACTION. 105 



of refraction, the last ray subjected to the influence of interference would 

 be the ray jo m Avhich passes through the an- 

 gle a. It falls on the inferior surface ec? at 

 the middle point m, and, being reflected in the 

 direction m of, reaches the eye at o' : every 

 more oblique ray, such as gm, falls beyond 

 the face ab, meets the vertical face ac, and 

 contributes nothing to the coloration, which 

 depends on the distance of the two faces a b 

 and c d. In order to comprise the ray ff m 



within this interval, a b should be prolonged to a' on the side of the 

 incident, and to b' on the side of the reflected rays. But as it termi- 

 nates at a and b, its field of coloration is confined within the limits mp, 



mo'. Now the angle amp, the sine of which is — = (because abed 



v5 



is a square) does not amount to 27°, and this is an opening too small to 



admit the manifestation of any change whatever in the tints. 



If the refraction (which precedes the reflexion) tends, as is evidently 

 the case, to enlarge the field of coloration, it has a still greater tendency 

 to diminish the effect of the change of the tints. It may therefore be 

 considered certain, that the integrant particles of which bodies are 

 composed cannot, in general, favour the play of the varying colours, 

 unless, in defiance of all other observations taken collectively, we assign 

 them a very considerable magnitude. 



After the foregoing reflexions there remains, so far as I can see, but 

 one point to be cleared. It being once admitted that the field of 

 coloration of the integrant molecules is confined within narrow limits, 

 how then, it will be asked, do bodies appear coloured in every direc- 

 tion ? In general the molecules hold, in the bodies which they form, 

 all sorts of positions, and are divided, relatively to the eye, into two 

 classes ; those of the one presenting their faces, and those of the others 

 their angles toward the observer. The first are those which colour 

 bodies ; the second are those which in one position of the mass con- 

 tribute, but in another do not contribute, to its coloration. In short, 

 the eye is always in the field of coloration of a vast number of particles. 

 When the field of one particle disappears, it is replaced by the field of 

 another ; so that the entire system always continues of a certain colour. 

 Symmetrical arrangements present an exception, and we have already 

 treated of these in the preceding paragraph. 



Metallic Colours. 



According to painters there are but three primitive colours, i-ed, yelloWi 

 and blue. By combining these tints in various proportions with black 

 and white they form the others. In richness and variety their produc- 



