a new Photographic Process. 437 



The very faint negative picture, produced as above, will, if 

 plunged into a weak solution of the sulphate of the protoxide 

 of iron, be immediately changed into a positive one; the 

 shadows being formed by a deposit of prussian blue, more 

 copiously over those parts which were shaded than those on 

 which the sun had more influence. The picture thus formed 

 is however somewhat indistinct, but by looking through the 

 paper it is seen that every part is faithfully preserved. If in- 

 stead of the proto-salt the persulphate is used, a very intense 

 blue negative picture of considerable interest results. In this 

 case the deposit of prussian blue takes place over the solarized 

 portions of the paper, the precipitation being relatively ac- 

 cording to the quantity of " energia " which has acted upon 

 the preparation. The lighter parts of these photographs are 

 at first yellow, and if left in this state they are liable to be- 

 come blue; but if soaked for a few minutes in a solution of 

 carbonate of soda, the yellow colour is removed, and the pic- 

 ture is a white and an intense blue. These drawings, as you 

 will perceive from the specimens sent, will not serve as origi- 

 nals from which copies might be taken, they do not possess 

 transparency. It is curious, however, to mark the develop- 

 ment of a positive image on the back of each negative picture, 

 which, though faint, has much the air of a mezzotinto engra- 

 ving. 



The different action of the two sulphates of iron is remark- 

 able, and it hardly admits of a satisfactory explanation. It 

 would appear that the ferrocyanate of potash, spread upon 

 paper, undergoes some change which prevents its precipitating 

 the proto-sulphate, by long exposure to sunshine ; but with 

 this salt alone I have never, in this way, been enabled to get 

 more than an image of the body obscuring the paper ; I there- 

 fore regard the bichromate of potash as accelerating merely 

 the change on the ferrocyanate. Papers spread with the fer- 

 rocyanate alone are affected precisely in the same manner if 

 plunged into a solution of the sulphate of the peroxide. In 

 both cases with the chromo-cyanotypes, the unchanged parts 

 of the picture precipitate prussian blue, and hence we have in 

 the one case a positive picture on the right side of the paper, 

 and in the other case a positive image on the wrong side of 

 the paper. I find, upon dipping a photographic picture 

 formed on paper, simply spread with the bichromate of potash, 

 into the per-solution, that every part is immediately removed, 

 and the paper left perfectly white. From this it would appear 

 that the oxide of chromium formed over the most exposed 

 parts is dissolved off, leaving the ferrocyanate of potash to 

 exert its influence on the salt of iron, which it does with greater 



