NATURAL PHILOSOPHY. 177 



ited. It gives a very rich scale of tints, from the lightest to the deepest, ac- 

 cording to the duration of the exposure to the light. The paper is prepared 

 by placing it for a few minutes on the surface of the bath and immediately 

 drying it. When sufficiently impressed by the light, through the negative 

 which it is desired to print, the image is fixed by the immersion for some time 

 in pure water. At this moment the print has not attained its perfection ; by 

 immersing it in a strongish solution of alum, the color is heightened in a re- 

 markable manner. M. de Beauregard obtained the yellow color by impreg- 

 nating the paper with a solution of bichromate of potash, and a prolonged ex- 

 posure to light causes this color to change to green. He fixes the print by 

 washing in common water and then by immersion in a solution of alum. The 

 bichromate of potash may also be usefully employed for giving black tints, 

 which may be carried to a very great intensity, without any salt of silver ; a 

 most important result in regard to economy hi the production of positives. 

 The mode of treatment is this : After removing from the frame the print ob- 

 tained on paper impregnated with the bichromate of potash, it is immersed for 

 a few minutes in pure water. Then it is passed through a solution of proto- 

 sulphate of iron. The paper is washed again, and then loses all trace of the 

 picture. But on plunging the paper into a bath of gallic acid, the print is de- 

 veloped, and assumes a blue -black color, the intensity of which may be in- 

 creased by using an infusion of logwood. One of the most interesting points 

 of the process of M. de Beauregard, and that which has most strongly excited 

 attention, is the different coloring upon the prints. The process by which he 

 obtained these various colors, which he has succeeded in producing on the 

 same paper by a single exposure to the light in the printing-frame, consists in 

 impregnating the paper with two mixtures successively, taking care to dry the 

 paper after the employment of each mixture. The first mixture is formed by 

 a solution of permanganate of potash with the addition of tincture of turn- 

 sol. The second mixture is formed of ferro-cyanide of potassium acidulated 

 with sulphuric acid. The paper thus prepared must lastly be subjected to a 

 bath of nitrate of silver. After the impression has been obtained, the paper is 

 first washed in pure water, then immersed in a weak bath of hyposulphite of 

 soda ; finally after a fresh washing, the colors are brought out vividly hi a 

 bath of neutral gallate of ammonia. 



It remains to give an explanation of the phenomena presenting themselves 

 in the production of these prints. They are referable to a physical and chem- 

 ical reaction produced by solar light on the different bodies and salts above 

 indicated and employed for the first time hi photography. M. Beauregard 

 advances a theory in reference to this discovery, which is, that the different 

 luminous rays impress the collodionized glasses in a manner precisely similar 

 to that which is necessary for the exact reproduction of the natural colors ; so 

 that the negative well brought out by light, possesses in itself, and by the 

 effect of the radiations of the different rays of the spectrum, the relative and 

 proportionate intensities proper to develop, on the positive paper prepared by 

 his process, the natural colors of the model. 



If this theory should prove true, the problem of obtaining color directly by 

 the agency of light would be solved. Jour. Pliot. Soc. London, 



8* 



