546 E. BECQUEREL ON THE CONSTITUTION 
between H and G. The lines H are therefore those which first 
appear. 
Fig. 5 represents the action of the spectrum on paper prepared 
with bromide of silver, with the omission of the lines. 
If we use different sensible substances the chemical spectrum 
is not of the same length, and extends a greater or less distance 
on the side of the red. It is thus that Sir J. Herschel, when 
studying the action of the iodide of potassium on paper prepared 
with a salt of silver already acted upon, saw an action produced 
beyond the red rays. 
I shall speak somewhat more in detail of the action produced 
by light on paper prepared by this method. 
We know that a sheet of paper done over with a salt of silver 
generally becomes black when exposed to the light; but if this 
paper is afterwards washed in a solution of an alkaline iodide, 
such as the iodide of potassium or of sodium, and if it is exposed 
when still moist to the action of the light, it directly becomes 
white: this reaction even takes place between the alkaline iodide 
and the metallic silver, under the influence of the solar rays pot- 
ash is liberated, and iodide of silver is formed. If, as Sir J. Her- 
schel did, we prepare a paper with a strong solution of acetate of 
lead, of bromide of potassium, then of nitrate of silver, this paper 
then done over with bromide and acetate of silver will become 
black in the light; but if, when it has become black and has im- 
bibed a weak solution of iodide of potassium, it is exposed in the 
spectrum, it directly becomes white towards the most refrangible 
extremity, and this action even continues beyond, whilst under 
some circumstances the paper will blacken more in the least re- 
frangible parts; this action then extends as far as the red, and 
even beyond. In order that this experiment should succeed, it 
is necessary that the solution of alkaline iodide should be very 
much diluted. When it is stronger the paper begins to become 
white in the least refrangible rays, whilst between the white and 
obscure part there is a neutral space. With another dose of iodide, 
the neutral line approaches the red; in short, when there is a 
sufficient solution of iodide, the paper becomes of a distinctly 
white nearly toward the red; with a very strong solution, the 
paper even becomes white in the dagk. 
In this reaction there are some very complicated effects, for 
not only does the iodide of potassium exercise an action on the 
