548 E. BECQUEREL ON THE CONSTITUTION 
it continually becomes weaker, so that towards M the coloration 
is scarcely sensible. When the action of the light is prolonged, 
whilst the spectrum is projected on paper prepared in this man- 
ner, we still see the image of the same lines which we have indi- 
cated, but feebly marked in white, for it is a paper producing 
inverse effects. If it is soaked in a dilute alcoholic solution of 
iodine, the light spaces become blue from the reaction of the 
iodine on the starch; the lines are then more visible. 
We have also used a paper done over with a layer of guaia- 
cum resin, which becomes blue in the light. It gives the same 
results with respect to the rays as the other sensible substances, 
only in a weaker degree, for it becomes blue but slowly under 
the influence of the solar rays. The blue colour which this resin 
assumes under the influence of the light arises from its oxidation, 
since all the oxidizing bodies, such as chlorine, bromine, &c., 
give it this colour. A greater degree of sensibility may be given 
to this resin by soaking the paper in a weak solution of nitrate 
of silver before putting a layer of guaiacum on it; the results 
obtained in the spectrum are more apparent, but they do not 
arise from the action of the solar rays on the pure guaiacum. 
M. Biot has shown that the matter which is soluble in water 
in this resin does not become coloured in the light, whilst that 
which is soluble in alcohol alone took the blue tint when it was 
subjected to this influence. Having therefore taken an alcoholic 
solution of guaiacum after this substance had been several times 
boiled in water, I covered a very white paper with it, which I 
exposed in the spectrum from two to three hours. The colora- 
tion in blue showed itself beyond the violet rays, as in fig. 8 ; 
the maximum was between M and N. These lines were feebly 
marked on the paper; it is probable that by leaving it longer 
exposed in the spectrum a stronger action would have taken 
place. 
We know that in light there are rays which restore the guai- 
acum that had become blue to its primitive colour, which is 
yellowish. These rays, as Wollaston has shown, are situated 
towards the least refrangible part of the solar spectrum. Instead 
of acting upon the guaiacum rendered blue by the action of 
light, we operated upon some resin coloured blue by an aque- 
ous solution of chlorine. Then, by exposing this paper in the 
spectrum, we had (fig. 9), after some hours, a white space ex- 
tending from the red as far as the violet. This action arises from 
