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.* 



3. 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 

 is, I think, better to take those on thick paper, because, though the 

 light is not so easily transmitted, yet the impression of the engraving 

 is much bolder, so that a more distinct delineation is given by the 

 photographic process. 



Camera Obscura. — The use of the camera obscura for photo- 

 graphic purposes, has been described by Mr. Talbot. Though repre- 

 sentations may be got in this way, yet, so far as I have found, they 

 have not the minute distinctness of those got by the method already 

 noticed. Owing to the interference of the lens, the light does not act 

 nearly 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, 



* Instead of purchasing lunar caustic of commerce, a cheaper method of procuring 

 it is to dissolve pure silver in nitric acid diluted with its own bulk of water, taking 

 care to have in the vessel more silver than the acid can dissolve ; and after it has 

 taken up as much as it can, to dilute the solution with four or five parts of water, or 

 thereabouts, according to the color required. 



Vol. XXXVII, No. 1.— July, 1839, bis. 23 



