178 ANNUAL OF SCIENTIFIC DISCOVERY. 



most faithfully represented," but they " slowly faded out, and became 

 eventually one uniform reddish tint." In M. Niepce's early experi- 

 ments, made in 1851 and 1852, and published in three memoirs on 

 heliochromy in the Comptes Rendus, he obtained his colored pictures 

 by preparing a bath composed of the deuto-chloride of copper ; but in 

 his more recent researches he has discovered a very remarkable 

 action of the chloride of lead in the double relation of white and the 

 duration of the color of the image submitted to the influence of light. 



The colors of the landscape have been accidentally produced in 

 the operations of photography. Mr. Raymond, a French artist, when 

 developing a picture on collodion by a combination of pyrogallic and 

 acetic acids, exposed it to light without washing it, and observed it 

 transform itself quickly into a positive, assuming, with more or less 

 perfection, the colors of the model. The best picture he obtained 

 required a quarter of an hour for its development. It preserved its 

 colors by an exposure to the air for some months, and was not com- 

 pletely effaced at the end of two years. 



Several photographers have observed colors in their landscapes ; 

 but they are the colors of thin plates, and have no relation whatever 

 to the colors of nature. 



In a memoir published two years ago, M. Niepce has shown how to 

 produce red, green, violet and blue photographs. A fine blood-red 

 color is produced by a solution of twenty parts of nitrate of uranium 

 in one hundred of water. The paper, after being fifteen or twenty 

 seconds in this solution, is dried in the dark. It is exposed for eight 

 or ten minutes under a negative, washed for a few seconds in water 

 at 50 or 60 Cent., and then immersed in a solution of red prussiate 

 of potash, composed of two parts to one hundred of water. It then 

 has a fine blood-red color, and must be washed repeatedly till the 

 water is limpid. A green color is obtained by immersing the preced- 

 ing red paper in a solution of nitrate of cobalt. When taken out 

 and dried at the fire without washing, its color will be green. It is 

 then fixed by putting it for a few seconds in a solution of sulphate of 

 iron and sulphuric acid, each four parts in one hundred of water. It 

 is then passed once through water and dried. A violet picture will 

 be obtained, with the paper prepared as above, with nitrate of 

 uranium. When it is taken from beneath the negative, it is washed 

 in warm water, and developed in a solution of chloride of gold, of 

 one-half part to one hundred of water. When it has taken a fine 

 violet color, it is washed several times in water and dried. In order 

 to get a blue picture, the paper is prepared with a solution of prus- 

 siate of potash, twenty parts to one hundred of water. It must be 

 taken from beneath the negative when the insulated parts have a 

 light blue tint, and then put for five or ten seconds into a cold 

 saturated solution of bichloride of mercury. When washed once in 

 water, and a cold saturated solution poured upon it of oxalic acid, at 

 the temperature of 50 or 60 Centigrade, it is then washed three or 

 four times and dried. 



CELESTIAL PHOTOGRAPHY. 



At the Leeds meeting of the British Association, Mr. De La Rue 

 exhibited binocular lunar pictures, which, when combined in the 



