8 HISTORY OF PHOTOGRAPHY.

minutes are sufficient to produce the full effect; in the shadeseveral hours are required; and light transmitted throughdifferent coloured glasses acts upon it with different degrees ofintensity. Thus, it is found that red rays, or the commonsunbeams, passed through red glass, have very little actionupon it; yellow and green are more efficacious; but blue andviolet light produce the most decided and powerful effects.

“ When the shadow of any figure is thrown upon the preparedsurface, the part concealed by it remains white, and the otherparts speedily become dark. For copying paintings on glass,the solution should be applied on leather; and in this case if ismore readily acted on than when paper is used. After thecolour has been once fixed on the leather or paper, it cannot beremoved by the application of water, or water and soap, and itis in a high degree permanent. The copy of a painting or theprofile, immediately after being taken, must be kept in anobscure place; it may, indeed, be examined in the shade, but inthis case the exposure should be only for afew minutes: by thelight of candles or lamps, as commonly employed, it is notsensibly affected. No attempts that have been made to preventthe uncoloured parts of the copy or profile from being actedupon by light, have as yet been successful. They have beencovered by a thin coating of fine varnish, but this has notdestroyed their susceptibility of becoming coloured; and evenafter repeated washings, sufficient of the active part of thesaline matter will adhere to the white parts of the leather orpaper to cause them to become dark when exposed to the raysof the sun. Besides the applications of this method of copyingthat have just been mentioned, there are many others; and itwill be useful for making delineations of all such objects as arepossessed of a texture partly opaque and partly transparent.The woody fibres of leaves, and the wings of insects, may bepretty accurately represented by means of it; and in this case it1s only necessary to cause the direct solar light to pass throughthem, and to receive the shadows upon leather.

“The images formed by means of a camera obscura have beenfound to be too faint to produce, in any moderate time, an effectupon the nitrate of silver. To copy these images was the firstobject of Mr. Wedgwood in his researches on the subject; andfor this purpose he first used nitrate of silver, which wasmentioned to him by a friend as a substance very sensible tothe influence of light; but all his numerous experiments as totheir primary end proved unsuccessful. In following theseprocesses, I have found that the images of small objects, pro-duced by means of the solar microscope, may be copied without