Page:Scientific Memoirs, Vol. 1 (1837).djvu/117

of refraction, the last ray subjected to the influence of interference would be the ray $pm$ which passes through the angle $a$. It falls on the inferior surface $cd$ at the middle point $m$, and, being reflected in the direction $mo'$, 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 $cd$. In order to comprise the ray $gm$ within this interval, $ab$ should be prolonged to $a'$ on the side of the incident, and to $b'$ on the side of the reflected rays. But as it terminates at $a$ and $b$, its field of coloration is confined within the limits $mp$, $mo'$. Now the angle $omp$, the sine of which is ${\frac {1}{\sqrt {5}}}$ (because $abcd$ 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 direction? 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 contribute, 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-