69 
1908-9.] A Question in Absorption Spectroscopy. 
positive one, which is regarded as fixed. Let a second doublet of co-ordin- 
ate x ± and different free period commence vibrating near the first. It 
will act on the first electron, and this action may be represented by intro- 
ducing a new term lx 1 in equation (1), l being small in comparison with 
k. On account of their rapidity we may regard l as a constant as far as 
the light vibrations are concerned. Equation (1) now runs, 
m^ + h^ + Jcx + lx^ 0 ( 2 ) 
at “ at 
We have a similar equation for the second doublet. On solving, we find 
that the mutual action has slightly altered the periods of both vibrations. 
As an absorption band corresponds to a free period, absorption bands might 
thus affect one another. The effect of the mutual actions of the electrons 
on the theory of dispersion and absorption has been fully worked out by 
one of the authors.* 
On the other hand, we should expect this shift to be too small to observe. 
The absorption bands of organic colouring matters usually appear at 
different positions in the spectrum when different solvents are used. Thus 
the fuchsine band in water is at 540 /u/x, while in alcohol it is at 555 /a/a. 
Kundt stated the law that the absorption band is farther to the red the 
greater the dispersion of the solvent; but investigation has shown that 
this law is as often as not disobeyed. It is nevertheless still given 
in the text-books. This dependence of the position of the absorption 
band on the solvent — the “ Kundt effect ” — is supposed to be due to a 
physical action between the molecules of the solvent and the molecules 
of the colouring matter. If changing the whole medium in which the 
doublet is placed displaces the band only by something of the order 
of 15 fxfj., then adding a small quantity of another colouring matter 
in the proportion of at the most 5 gms. per litre should have no 
appreciable effect. 
The apparatus was a spectrophotometer which has been described in an 
article in the Phil. Mag.f This instrument has two slits, one about 7 mm. 
vertically above the other. Both slits are illuminated by the same source, 
and the liquids, the absorption of which is to be compared, are placed one 
in front of each slit. The nicol eyepiece in the telescope may be replaced 
* R. A. Houstoun, “ Untersuchungen fiber den Einfluss der Temperatur auf die Absorp- 
tion des Lichtes in isotropen Korpern,” Diss. Gottingen, 1906, Ann. d. Phijs. (4), xxi. (1906), 
p. 535. 
t Ibid ., “A New Spectrophotometer of the Hiifner Type,” Phil. Mag., February 
1908. 
