188 ANNUAL OF SCIENTIFIC DISCOVERY. 



ble in water after a short exposure to sunlight. This principle is capa- 

 ble of application to photography in many ways, one of the most ob- 

 vious of which is to attach to paper a suitable tissue, and cover it with 

 bichromated gelatin having a pigment mixed with it ; expose this tis- 

 sue to light, under a negative, and then wash away those portions of 

 the coating not affected by the light. The exposed parts, having 

 become insoluble, remain attached to the paper, and so produce a pic- 

 ture. The mixture of gelatin consists of one part of a solution of 

 bichromate of ammonia (containing one part of tliQ salt in three parts 

 of water), two parts gelatin, one part sugar, and eight parts of water, 

 with coloring matter added to produce the depth of tint required. 

 The pigment used is Indian ink, either alone or mixed with indigo and 

 carmine. 



The tissue is formed by coating a plate of glass or other smooth 

 surface, first with collodion, and then with the colored gelatin 

 mixture above described : the two films unite, and, when dry, may be 

 separated in a sheet from the surface they were formed on. By this 

 means a pliant tissue is obtained, which may be handled like paper, 

 and may either be used in large sheets or cut up into pieces of any 

 convenient size. The tissue, prepared in the manner described, cor- 

 responds with sensitive paper, and, with proper appliances, the prep- 

 aration of it need not be more troublesome than the double operation 

 of albumenizing and exciting paper in the usual way. The tissue is 

 much more sensitive to light than ordinary sensitive paper, and pro- 

 portionately more care must be exercised to guard it from the action 

 of light other than that which acts upon it while in the printing-frame. 

 Like sensitive paper, too, it is better used soon after its preparation. 

 The printing is done in the usual wav, the tissue taking the place of 

 sensitive paper, the colloclionized surface being placed next the nega- 

 tive. The sensitiveness of the tissue may of course be varied by 

 varying the proportion of the components of the gelatinous part of 

 the tissue ; but with the mixture given, the time of exposure required 

 is only or 4 of that which would be usually given with highly sensi- 

 tive albumenizcd paper. 



"The proper time for exposure can be determined pretty accu- 

 rately, after a few trials ; for, although there is not the same means of 

 judging of the progress of the printing as in the ordinary process, 

 yet there is a far wider range between under and over exposure than 

 in silver printing. It is no exaggeration to say that you may expose 

 one piece of tissue twice as much as another, and yet obtain a good 

 print from both ; not perhaps quite so good as between the two ex- 

 tremes, but yet much more passable than would be the case with 

 silver prints under and over exj>; >sed to the same extent. On taking 

 the tissue from the printing-frame the image is iaintly visible, and the 

 next st"j> i:i the process i> to m omit the tissue, with the collodionized 

 i'a'-e down, upon a piece of paper, or any other suitable material, to 

 a ;t i ; a s lo^ort during de\v! >;rn "at, and sometimes to form the basis 

 of the picture, which m.iy, if we please, remain permanently attached 

 to this support, or may, if thought better, be afterwards transferred. 

 There are several ways of mounting the tissue, and several adhe- 

 sive substances may be used for the purpose, such as starch or a 

 solution of India-rubber and dammar in benzole. 



