1 66 Royal Society :— Mr. Talbot on 



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

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

 upon which the sun is shining. When the aperture amounts to 

 one-third of the focal length, and the object is very white, as a 

 plaster bust, &c, it appears to him that one second is sufficient to 

 obtain a pretty good image of it. 



The images thus received upon the Calotype paper are for the 

 most part invisible impressions. They may be made visible by the 

 process already related, namely, by washing them with the gallo- 

 nitrate of silver, and then warming the paper. When the paper is 

 quite blank, as is generally the case, it is a highly curious and 

 beautiful phenomenon to see the spontaneous commencement of 

 the picture, first tracing out the stronger outlines, and then gra- 

 dually filling up all the numerous and complicated details. The 

 artist should watch the picture as it developes itself, and when in 

 his judgment it has attained the greatest degree of strength and 

 clearness, he should stop further progress by washing it with the 

 fixing liquid. 



The fixing process. — To fix the picture, it should be first washed 

 with water, then lightly dried with blotting paper, and then 

 washed with a solution of bromide of potassium, containing 100 

 grains of that salt dissolved in eight or ten ounces of water. After 

 a minute or two it should be again dipped in water and then finally 

 dried. The picture is in this manner very strongly fixed, and with 

 this great advantage, that it remains transparent, and that, there- 

 fore, there is no difficulty in obtaining a copy from it. -The Ca- 

 lotype picture is a negative one, in which the lights of nature are 

 represented by shades ; but the copies are positive, having the lights 

 conformable to nature. They also represent the objects in their 

 natural position with respect to right and left. The copies may 

 be made upon Calotype paper in a very short time, the invisible 

 impressions being brought out in the way already described. But 

 the author prefers to make the copies upon photographic paper pre- 

 pared in the way which he originally described in a memoir read 

 to the Royal Society in February 1839*, and which is made by 

 washing the best writing paper, first with a weak solution of com- 

 mon salt, and next with a solution of nitrate of silver. Although it 

 takes a much longer time to obtain a copy upon this paper, yet 

 when obtained, the tints appear more harmonious and pleasing to 

 the eye ; it requires in general from 3 minutes to 30 minutes of 

 sunshine, according to circumstances, to obtain a good copy on 

 this sort of photographic paper. The copy should be washed and 

 dried, and the fixing process (which may be deferred to a subse- 

 quent day) is the same as that already mentioned. The copies are 

 made by placing the picture upon the photographic paper, with a 

 board below and a sheet of glass above, and pressing the papers 

 into close contact by means of screws or otherwise. 



After a Calotype picture has furnished several copies, it some- 

 times grows faint, and no more good copies can then be made from 

 * Printed in Phil. Mag., Third Series, vol. xiv. p. 209.— Edit. 



