Dr Fyfe on Photography. 147 



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 light. For this pm'pose 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 depends, of coxirse, 

 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 dis- 

 tinct 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 impres- 

 sion by too long exposm'e, 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 thm 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 pho- 

 tographic purposes has been described by Mr Talbot. Though 

 representations 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, 



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 so- 

 lution with four or five parts of water, or thereabouts, according to the 

 colour required. 



