313 



Then take a sheet of iodized paper and wash it over with this gallo' 

 nitrate of silver, with a soft brush, taking care to wash it on the side 

 which has been previously marked. This operation should be per- 

 formed by candlelight. Let the paper rest half a minute, and then 

 dip it into water. Then dry it lightly with blotting-paper, and 

 finally dry it cautiously at a fire, holding it at a considerable distance 

 therefrom. When dry, the paper is fit for use. The author has 

 named the paper thus prepared Calotype paper, on account of its 

 great utility in obtaining the pictures of objects with the camera 

 obscura. If this paper be kept in a press it will often retain its 

 qualities in perfection for three months or more, being ready for use 

 at any moment ; but this is not uniformly the case, and the author 

 therefore recommends that it should be used in a few hours after it 

 has been prepared. If it is used immediately, the last drying 

 may be dispensed with, and the paper may be used moist. Instead 

 of employing a solution of crystallized gallic acid for the liquid B, 

 the tincture of galls diluted with water may be used, but he does not 

 think the results are altogether so satisfactory. 



Use of the Paper. — The Calotype paper is sensitive to light in an ex- 

 traordinary degree, which transcends a hundred times or more that of 

 any kind of photographic paper hitherto described. This may be made 

 manifest by the following experiment : — Take a piece of this paper, 

 and having covered half of it, expose the other half to daylight for 

 the space of one second in dark cloudy weather in winter. This 

 brief moment suffices to produce a strong impression upon the paper. 

 But the impression is latent and invisible, and its existence would 

 not be suspected by any one who was not forewarned of it by pre- 

 vious experiments. 



The method of causing the impression to become visible is ex- 

 tremely simple. It consists in washing the paper once more with 

 the gallo-^ nitrate of silver, prepared in the way before described, and 

 then warming it gently before the fire. In a few seconds the part 

 of the paper upon which the light has acted begins to darken, and 

 finally grows entirely black, while the other part of the paper re- 

 tains its whiteness. Even a weaker impression than this may be 

 brought out by repeating the wash of gallo-nitrate of silver, and 

 again warming the paper. On the other hand, a stronger impres- 

 sion does not require the warming of the paper, for a wash of the 

 gallo-nitrate suffices to make it visible, without heat, in the course 

 of a minute or two. 



A very remarkable proof of the sensitiveness of the Calotype paper 

 is afforded by the fact stated by the author, that it will take an im- 

 pression from simple moonlight, not concentrated by a lens. If a 

 leaf is laid upon a sheet of the^paper, an image of it may be obtained 

 in this way in from a quarter to half an hour. 



This paper being possessed of so high a degree of sensitiveness, 

 is therefore well suited to receive images in the camera obscura. 

 If the aperture of the object-lens is one inch, and the focal length 

 fifteen inches, the author finds that one minute is amply sufficient in 

 summer to impress a strong image upon the paper of any building 



