OF THE SOLAR SPECTRUM. 543 
means of the measures taken upon the same plate; it is there- 
fore necessary to take a great number of impressions by ex- 
_ posing the plates, during different times, with different apertures 
of the shutter, in order to obtain fine lines. If the plate remains 
: exposed during an hour in the spectrum, we see the action ex- 
tend from the limit of the green and of the blue as far as P 
_ beyond the violet (fig. 2). 
: A longer time does not give any action beyond, we merely 
_ perceive a species of second spectrum from the green as far as 
} the red; but it is the spectrum of the continuing rays, which 
merely act because the plate exposed during a certain time to 
the action of the solar spectrum is in a small degree impressed 
by the diffused light which always accompanies the spectrum. 
The action of the exciting rays, that is to say of the rays which 
are ordinarily considered, and which commence and continue a 
chemical action, ceases with regard to the simply iodized plates 
towards the limits of the indigo at G; but with respect to the 
plates which are iodized and aan ards exposed to the vapour 
of chlorine or of bromine, it goes as far as F in the extreme blue 
_ by the side of the red. 
_ The spectrum of the continuing rays does not extend beyond 
the extreme red, for its action goes on diminishing as far as this 
extremity ; but it is especially with the simply iodized plates that 
_ it shows itself best. In fact, these plates being less sensible than 
_ others, the diffused light which accompanies the solar spectrum 
does not suffice to impress them enough at first. In order to 
"recognise the existence of these rays, it is Hecessary to impress 
these plates before the experiment, by exposing them some 
seconds to the daylight; I have also shown in a memoir that 
the action of the continuing rays was so much more energetic 
in as far as the substance was more impressed, but only to a 
_ certain limit. 
Figure 2 represents the spectrum with all its lines, drawn 
| pn to experiments made with different plates iodized and 
then exposed to the vapour of the chloride of iodine. The por- 
“tion of the image which first appears is that which is contained 
| between H and G; the maximum of action is then nearly be- 
tween these two Haves If the action is left to continue, it does 
yot extend beyond F, and indeed it suddenly ceases nearly at 
is ray, but extends, diminishing by degrees in intensity as far 
Z beyond the visible rays. If these plates remain exposed 
VOL, Ill. PART XII. 20 
