PRODUCED BY ELECTRO-CHEMICAL ACTION. . 107 
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 sub- 
stances 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*. 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 New- 
ton’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 eolours 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 
phznomena connected with thin lamine, because these lamine 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 isin 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 com- 
position 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 green+.” 
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 plausi- 
ble, 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 de- 
stined to be reflected in the interior of the same substance, escape undi- 
minished ? If the phenomenon be incomplete in respect to transmission, 
it will be equally so in respect to reflexion, and the tint formed will be 
* Optics, Book II. part 3. prop. 5. 
+ Biot, Traité de Physique, vol. iv. p. 127. 
