CHAP, vni.] HELIOGRAPHY PHOTOLITHOGRAPHY. 321 



Fraunhofer; the violet tint continues beyond H, gradually becoming 

 paler. On continuing the action of the spectrum, the tints darken 

 and the image eventually takes a metallic gloss ; the colours have then 

 disappeared." 



The colours thus obtained could be preserved for some time in the 

 dark ; but they disappeared in daylight, and M. Becquerel could not 

 succeed in fixing them. 



It is a curious thing that white comes out black on the plate ; but 

 by submitting the latter to a temperature of 80 to 100, the white 

 light produces a white impression. 



By placing a coloured engraving on the chloridized plate, M. 

 Edmond Becquerel also obtained the reproduction of the colours of 

 the picture by a sufficiently long exposure to the solar light; but he 

 had to interpose a screen of sulphate of quinine to impede the action 

 of the ultra-violet rays, which would have given the whole picture a 

 grayish tint. 



M. Niepce de Saint-Victor, improving on M. Becquerel's method of 

 working, succeeded in reproducing the colours of pictures and even 

 in obtaining black in conjunction with the other colours. The 

 sensitized coating then requires a particular preparation. The blue- 

 violet tinted surface is covered with a varnish of dextrine and chloride 

 of lead. " I have reproduced by contact," he says, " a coloured en- 

 graving representing one of the French guards : the different colours 

 of the uniform were reproduced ; the black hat, as well as one of 

 the gaiters (the other had been cut away and covered over with white 

 paper), impressed the plate very distinctly, giving a more or less dark 

 tint according to the preparation of the plate. The cutting-out showed 

 white." 



M. Niepce de Saint-Victor also found that the greater or less 

 concentration of the solution of the chloride used in preparing the 

 sensitized plate, influences the development of different colours. With 

 a weak solution, yellow comes most readily ; by augmenting progres- 

 sively the dissolved chloride, he obtained blue-green, then indigo, then 

 violet; and finally the less refrangible colours, orange and yellow, 

 require the most concentrated solution. Another interesting result is 

 this : the metallic chlorides exercise an analogous influence, or rather 

 one depending on the colour given by each of them to the flame of 

 alcohol. Hence, if we add to the solution a certain quantity of 



Y 



