Miscellanies. ' 177 
which can be applied to the paper on one side by means of a brush. 
‘Paper thus prepared is white; it has the advantage of being easily 
prepared, and of giving, on exposure to light, a uniform ground which 
is of a brownish color.* 
2. Methods of taking the Impressions. 
From what has been already stated, it must be evident that the most 
direct mode of taking the impressions is, by placing on the paper the 
object, the delineation of which is wished, and then exposing it to the 
light. For this purpose it ought to be kept as close as possible on the 
paper, and the best method of doing so is to place it in a frame with 
glass in front, and a stuffed cushion behind it. The time required de- 
pends, of course, on the intensity of the light, and the density of the 
object; and it is of the utmost consequence to take care that it is long 
enough exposed, and that, at the same time, the exposure is not too 
long continued, for if not long enough, though the outline will be 
given, yet the representation will not be distinct in all its parts; 
Whereas if too long continued, the fainter parts begin to darken, 
and the representation is indistinct. The time required must be 
found by practice. In bright sunshine one minute will be sufficient 
for some objects: when there is no sunshine an hour or two may be 
required, and in this case there is little or no danger of destroying the 
impression by too long exposure, as the light is not of sufficient in- 
tensity to darken too much the fainter parts. 
Impressions from Engravings may likewise be got in the same 
Way; and for this purpose, instead of using those thrown off on thin 
Paper, by which it is supposed the light is most easily transmitted, it 
* Tthink, better to take those on thick paper, because, though the 
light is not so easily transmitted, yet the impression of the engraving 
18 much Bolder, so that a more distinct delineation is given by the 
Photographic process. 
Camera Obscura.—The use of the camera obscura for photo- 
Staphic purposes, has been described by Mr. Talbot. ‘Though repre- 
Sentations may be got in this way,.yet, so far as I have found, they 
ave not the minute distinctness of those got by the method already 
noticed. Owing to the interference of the lens, the light does not act 
Rearly so powerfully on the paper, as when it has to permeate merely 
a frame of glass. ‘The same is the case when the light is reflected, 
7 creeper tne 
‘ "Instead of purchasing lunar caustic of commerce, a cheaper method of procuring 
tis to dissolye pure silver in‘nitric acid diluted with its own bulk of water, taking 
“are to have in the vessel more silver than the acid can dissolve ; and after it has 
*8 Up as much as it can, to dilute the solution with four or five parts of water, or 
ired 
