4'86 BAUON vox TVREDK ON THE ABSOnPTIOX 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 = y^y of the length of the waves of red 

 light absorbs little light, or is translucid, and absorbs all the colours 

 with an almost equal power, that is, appears colourless. 



Let us now suppose 2 b 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 trafishicid, and that its colour must fall into the red. 



Let us now move fig. 2 a little further ; suppose, for example, that 

 2 6 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 nontransjmrent, 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 r, 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 bodies ; 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 r a, r a . (1 — r)-, r a (I — r)^ . r^ . . . etc., in the same manner 

 as we reckoned before the resultants of the transmitted rays, we obtain for their 

 intensity 



VI -I- 2 (I — 2 r) cos 2 ^ — + (1 — 2 r)2 

 A'=ra. , ^. .... (7) 



-yr.:- " 26 



2 r^ cos 2 ^ — + r* 



If this expression be differentiated in relation to — , it is evident that A' be- 

 comes a maximum when — is 0, 1, 2, 3, 4 and a minimum when — 



?k ' ' ' ' 3i 



is i, 2) 5-, I • • •, «• c- under the same circumstances as A. Hence it follows, that 

 Avhat has been said in respect to the transmitted light holds good also for the 

 reflected light ; s(. that the natural colour of bodies is explained in the san7e 

 manner for reflected light as foi- transmitted Hght. 





