542 - E, BECQUEREL ON THE CONSTITUTION 
which do not begin a chemical action, but which continue it if 
it is once begun; that is to say, which do not act except upon 
a substance already impressed; and these rays extend, for each. 
substance, from the limit of the least refrangible chemical rays, 
that is to say, nearly from the blue to the red. 
As I intended to study the properties of the chemical rays, as 
well as those of the luminous rays, I had to ascertain whether 
the chemical spectrum was continuous on the different impress- 
ible substances, or if, as in the case of the luminous spectrum, 
it was interrupted by more or less thick, black lines. I was led 
to this curious result, that the chemical spectrum has the same 
lines as the luminous spectrum, provided we only consider the 
parts of the same degree of refrangibility of these two radiations. 
To make this fact evident, I took a frame capable of receiving 
another, on which I stretched different papers covered with im- 
pressible substances, or surfaces of different nature. By using 
a plate of silver iodized, according to M. Daguerre’s method, 
and by exposing it a minute or two in the frame, placed at the — 
distance above mentioned, so as to receive the spectrum by pro- 
jection, and afterwards subjecting it to the mercurial vapour, we 
see that the plate has been more or less impressed, according to 
the intensity of the chemical action of the rays; generally it will 
be from the indigo to within a certain distance of the extreme 
violet. We then see a great quantity of lines, as in the lumi- 
nous spectrum; and by drawing these lines with their position 
and respective sizes, we find that they correspond to those of 
the luminous spectrum, at least in the visible parts; the lines 
which are best seen and which appear the strongest, are the two — 
marked H. 
By using a plate of silver iodized, and afterwards submitted. 
to the vapour of chlorine or of bromine, which, as we know, gives * 
a very great sensibility to the iodide of silver, the same effects 
are obtained in a few seconds. If the plate remain exposed to 
‘the spectrum for a longer time, say a few minutes, the image 
becomes more marked and more extended beyond the violet, 
whilst in the vicinity of H the too long continued action of the 
chemical rays blackens all the plate and prevents the lines from 
being distinguished in this part. On the other side of H, beyond 
the visible parts, we find an infinite number of lines, which I~ 
will indicate further on in fig. 2. 
From what has just been said they cannot all be drawn by © 
