PRODUCED BY ELECTRO-CHEMICAL ACTION. ; 105 
of refraction, the last ray subjected to the influence of interference routs 
be the ray pm which passes through the an- 
gle a. It falls on the inferior surface ed at 
the middle point m, and, being reflected in the 
direction m o’, 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 ab 
and ed. In order to comprise the ray gm 
within this interval, a 6 should be prolonged to a! on the side of the 
incident, and to 5! on the side of the reflected rays. But as it termi- 
nates at a and 8, its field of coloration is confined Sets the limits m p, 
mo'. Now the angle omp, the sine of which is ~= (because abcd 
fe 
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, red, yellow, 
and blue. By combining these tints in various proportions with black 
and white they form the others. In richness and variety their produc- 
