AND ON THE ACTION OF LIGHT ON BODIES. 425 
contains chloride of lime. A little sulphuric acid is poured upon 
the chloride, and the larger vessel closed and allowed to stand 
for two or three days; at the end of that time it will be found 
that the smaller bottle contains either one or other, or perhaps 
both the chlorides of iodine. If the plate of silver, after having 
been exposed to the iodine vapours, be moved over this chloride 
for 20 or 30 seconds, it will then yield images in a few seconds, 
and always in the same time if the intensity of light remain the 
same, so that there is no insecurity at all. 
If the prepared plate be exposed a sufficient time in a camera 
obscura to the action of light, an image is produced in which 
those parts are light which in nature are dark, and vice versd, 
because the iodide of silver retains its original colour on those 
_ spots where the light did not act. This kind of picture, which 
is perfectly useless in an artistical point of view, I shall hereafter 
_denominate “ negative,” as Herschel has already proposed. 
_ Daguerre’s discovery is, that before the negative image has 
been formed, at a time when no kind of action can be seen to have 
taken place on the iodide of silver, a certain kind of influence 
has been exerted in such a manner that those parts acted upon 
by the light have acquired the property of condensing or pre- 
cipitating the vapours of mercury. The first point that inter- 
ests us is the fact, that light can produce in iodide of silver 
_ modifications which are not rendered visible to our eyes by 
_ ordinary means; and in this respect the beautiful discovery of 
Becquerel, jun. leads us a step further. Ifthe vapours of mer- 
cury are to be condensed by the iodide of silver, it is necessary 
that the action of the light shall have continued for a certain 
time; if this period has not been allowed, no mercury is pre- 
| cipitated, and it might appear as if the plate had undergone no 
| change whatever. Becquerel, however, exposed a plate thus 
| treated to the sun, under a red glass, and he then obtained the 
positive image formed of the mercurial vapours, when the action 
of the light had not been too long continued ; when, however, the 
plate had been allowed to remain under the red glass above a 
certain period, the negative image was produced. From this it 
follows, that although the camera obscura had acted only for 
a short time, its influence on the iodide of silver had taken place, 
for the ved rays were capable of continuing the action. 
_ Proceeding from the assumption that the chemical effects of 
light are combined with the violet and blue rays, Becquerel 
VOL. III. PART XI. 2F 
