448 PROFESSOR LUDWIG MOSER ON VISION, 
scura, and then held over mercury, which was gradually warmed. 
In this way the following facts were discovered. 
It will have been seen from the preceding general laws, that 
it is not necessary to place the plate at any particular inclination 
to the vapours of mercury, e.g. that of 45°, as is always stated ; 
the plates may be placed according to pleasure if the vapours 
can reach them. I have also discovered a fact, of which it is ne- 
cessary to be aware in these experiments, viz. that there is no 
action of mercurial vapours of a high tension which cannot be 
produced by those of a low one, if the times be proportioned. 
In forming an image according to Daguerre, it is not necessary 
to heat the mercury ; if the plate, when taken out of the camera 
obscura, be placed above mercury and allowed to stand for 
some hours, the picture will be produced with the same exacti- 
tude. The complicated actions of mercurial vapours that I shall 
have to describe succeed equally well when a high or low de- 
gree of tension is employed, only in the latter case they require 
a much greater length of time. It is important to know this 
fact, for in the previous experiments, when the mercury has 
been heated once, sometimes no image is formed; if the plate 
be then allowed to stand over the cold mercury for 12 or 24 
hours, and the operation has been well conducted in other re- 
spects, a correct picture will make its appearance. The true 
reason of this will appear from what follows. 
Let an iodized silver plate, which has remained the proper 
time in the camera obscura, be brought into the mercurial appa- 
ratus, and the mercury gradually heated; the ordinary image 
will be seen to form when the temperature is about 70° R.*; let 
the heat be then raised to 100° R. If the picture be now re- 
moved from the apparatus, it is fied, while the common ones 
may be easily wiped off. This picture may be strongly rub- 
bed; it loses a little of its intensity at first, but not afterwards. 
The images, however, cannot generally resist rubbing with moist 
materials, or with polishing substances. I have obtained these 
fixed representations in a very beautiful state, and with great 
minuteness of detail, even by heating the mercury to 60° as 
* The temperatures will vary in the different experiments, according to the 
manner in which the plate has been treated, according to the mass of mercury 
and the source of heat, the height of the plate above the mercury, its tempera- 
ture, &c. 
