486 BARON VON WREDE ON THE ABSORPTION OF LIGHT 
will equal 20. I now place fig. 2 on fig. 1, in such a manner that the 
red end may lie on 20, and see that the whole part of the curve which re- 
presents the spectrum lies near the maximum ; and from this I conclude 
that the body which makes 2 6 = 34, of the length of the waves of red 
light absorbs little light, or is ¢ranslucid, and absorbs all the colours 
with an almost equal power, that is, appears colourless. 
Let us now suppose 2 6 somewhat greater, for example = } of the 
length of the wave of red light; fig.-2 must then be so placed that 
the red end may lie on 4; we then see that the entire spectrum lies 
nearer to the minimum of the intensity, but that the violet end lies 
nearest to this minimum; from this we conclude that the body is but 
slightly translucid, and that its colour must fall into the red. 
Let us now move fig.2 a little further; suppose, for example, that 
2b is equal to half the length of the wave of green light; we find that 
the whole spectrum lies in the minimum of the intensity ; the body must 
according to that be nontransparent, when r is so great that the inten- 
sity of the minimum lies within the limit of our range of vision; in the 
contrary case it must appear black. 
If we continue to move fig.2 still further, that is, to suppose con- 
stantly increasing retardations, we obtain spectra in which the maximum 
of the intensity falls successively on the violet, blue, green, yellow and 
red, and in which the complementary colours are more or less absorbed. 
Let us now suppose the magnitude of 7, on which the real magnitude 
of the intensity, but not the condition of the maxima and minima, de- 
pends, to be also unequal on the different bedies ; we then easily per- 
ceive that we can imagine all degrees of the natural colours of the bodies, 
as well as their greater or less transparency, as originated in this 
manner *. 
* If we reckon the resultants of the reflected rays, the intensities of which 
(fig. 9) are ra, ra.(1—7r)*, ra(1—r)?.7?... etc., in the same manner 
as we reckoned before the resultants of the transmitted rays, we obtain for their 
intensity 
J 142.0 —2n cos 202+ (d—2r) 
AS 1. «enh tee (7) 
J 127 cos25 2h 4 1 f 
If this expression be differentiated in relation to a it is evident that A! be- 
comes a maximum when - is 0, 1,2, 3,4... ., and a minimum when ef 
is 3, 3, % 3 «++, i.e. under the same circumstances as A. Hence it follows, that 
what has been said in respect to the transmitted light holds good also for the 
reflected light; so that the natural colour of bodies is explained in the same 
manner for reflected light as for transmitted light. 
