OF THE SOLAR SPECTRUM. 545 
hardly distinct, for P is nearly the limit to which the chemical 
action extends. 
We have only described the most remarkable; there are a 
quantity of others which are represented, and probably a great 
number of fine lines which cannot be shown by projection. 
This is the extent of the chemical spectrum obtained with the 
iodized plates, that is to say from Z as far as X, fig. 3, this spec- 
trum containing the continuing rays from F as far as X. When 
we employ plates simply iodized, the action of the continuing 
_ [exciting ?] rays, instead of extending from Z as far as F, has its 
limits between O and P on one side, and on the other between 
G and H; the continuing rays begin between G and H at the 
boundary of these, and go as far as the red. 
Figures 4 and 5 represent the extent of the spectra obtained in 
these two circumstances. 
In the following manner we may observe the lines of the che- 
mical spectra produced during the action of the solar light on 
different sensible substances. For example, we take paper pre- 
pared with bromide of silver, chloride of silver, or any salt of 
silver whatever. It is placed upon a slate if we wish to act upon 
it whilst it is moist, or in a wooden frame in case it is to be ope- 
rated upon when it is dry. It is then put in the frame, so that 
_ the solar spectrum by projection shall be exactly at the focus of 
the lens. We then see the lines of the luminous spectrum, and, 
by allowing the action of the light to continue during an interval 
of time, depending on the sensibility of the salts, the image is 
Seen to form upon the paper. With these salts the effect is gene- 
Tally the inverse of the natural spectrum, that is to say that the 
parts corresponding to the chemical rays appear black on the 
paper, and that the lines remain white ; they are less easily distin- 
guished than upon the iodized plates, but, with a little attention, 
the lines of intermediate size are found in the same positions as 
_ formerly, and consequently in the same places as the lines of the 
ee 
luminous spectrum; thus the same figure (3) gives all the rays 
of the chemical spectra. The action of the continuing rays, of 
which I have already spoken, on these different salts is also 
seen; these rays extending from the least refrangible limit of the 
exciting rays as far as the red. 
With these substances the maximum of action does not occur 
at the same place in the spectrum; but it is always situated nearly 
202 
