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tinued before this effect comes on, an invisible impression is the 

 result, to develope which all that is necessary is to breathe upon 

 it, when it immediately appears, and very speedily acquires an 

 extraordinary intensity and sharpness, as if by magic. Instead of 

 the breath, it may be subjected to the regulated action of aqueous 

 vapour, by laying it in a blotting-paper book, of which some of the 

 outer leaves on both sides have been damped, or by holding it over 

 warm water. 



CHROMOTYPE. 



M. Ponton was the first to point out the photographic properties 

 of bichromate of potash. His process for preparing paper is as 

 follows : Immerse a well-sized paper in a saturated solution of 

 bichromate potash, and dry by the fire. It is of a fine yellow 

 colour, and keeps well in the dark. When exposed to the rays 

 of the sun, it becomes of a light brown ; and if an engraving has 

 been placed upon it, the resulting picture is negative. It is fixed 

 by soaking in water. Mr. E. Becquerel improved upon this process 

 by applying evenly over the paper a sizing of starch, and then 

 steeping it in the bichromate solution as before. The picture 

 having been taken, and the paper washed and dried, it is immersed 

 in a weak alcoholic solution of iodine, in which it remains some 

 time, and is then rinsed and carefully dried between blotting-paper, 

 without much heat. When wet, the shades of the picture are of a 

 fine blue ; but when dry, of a deep violet. If the picture, while 

 wet, is covered with a coating of gum, the colour is better preserved 

 and is more beautiful when dry. 



Mr. Hunt announced the process, which is termed the Chromo- 

 type, at the meeting of the British Association in 1843. It is not 

 sufficiently sensitive for the camera, but is valuable for copying 

 engravings, etc. Good writing paper is washed over with sulphate 

 of copper, in solution, about one drachm to an ounce of water ; 

 when dry, it is again washed with a strong, but not saturated 

 solution of the bichromate of potash, and again dried. The paper 

 may be preserved in this state for a considerable time. When 

 exposed to sunshine, it changes to a dull brown, and if checked 

 here, a negative picture is produced ; but if the action of light is 

 continued, the browning gives way, and the picture becomes posi- 



