which depend on the tenuity of plates are not to be traced on all classes of bodies; that they can be produced by those bodies only which are endowed with a certain degree of transparency; and that metallic substances are too opaque to be numbered among these. This is a positive fact, and ought therefore, without any regard to particular systems, to be entered in the register of science.
Gold and Copper.
It cannot be doubted, says Newton, that the colours of gold and copper belong to the second or the third order[1]/ To us they seem, on the contrary, to belong to the first order, that being the only one which includes tints of a metallic appearance. If we only recollect that the first colours of the scale are far from being distinct in the first of Newton's rings, we shall feel less surprised that it should be necessary to correct the classification of that great philosopher. The resemblance in question is, however, as we have observed already, very far from being perfect. The tints that come nearest to the yellow of gold are the blond colours Nos. 2 and 3: but these are evidently less yellow, and at the same time more compounded than the colour of gold; for they contain a tinge of green, which does not exist in the more decided colour of gold. Transparent gold-leaf appears green when held before the light: this fact has been classed by several persons among the phænomena connected with thin laminæ, because these laminæ are known to reflect a given colour, in the same position in which they transmit its complementary colour. However I will say with a great philosopher, that "there is in Newton's rings no yellow that has green for its complement: the colour transmitted is invariably the blue; and this fact accords with the construction given by Newton for the composition of colours. But extract from this blue (which is necessarily compounded) a certain number of violet and blue rays, such as may be absorbed by the substance of gold, and there will remain greenf[2].
It is a fact demonstrated by a great number of observations, that light in its passage through coloured substances is partially absorbed and extinguished. This fact not only renders Biot's explanation plausible, but warrants the supposition that light undergoes in reflexion a diminution analogous to that which takes place in its transmission. For if some of the rays destined to be transmitted are absorbed by the very substance of the gold, how can all the other rays, which are destined to be reflected in the interior of the same substance, escape undiminished? If the phænomenon be incomplete in respect to transmission, it will be equally so in respect to reflexion, and the tint formed will be
