438 Mr. Robert Hunt on Chromo-Cyanotype. 



energy than upon those parts on which the mixed salts of 

 chrome and ferrocyanogen remain unchanged. 



In subjecting paper thus prepared to prismatic analysis, the 

 space covered by the blue ray is the first which darkens, and 

 for some time the action is confined to this portion of the 

 spectrum ; but it extends at length, as before stated, from the 

 most refrangible limits of the green ray to a space a little be- 

 yond the visible violet rays. If, however, after a short expo- 

 sure to the spectrum, the paper is dipped into a solution of the 

 protosulphate of iron, a space which corresponds in extent to 

 the entire luminous spectrum is left free, or nearly so, of preci- 

 pitated prussian blue. If after an exposure of fifteen or twenty 

 minutes to the prismatic image, concentrated by means of a 

 lens, we wash the paper, without removing it, with the iron 

 salt, the precipitation appears to take place pretty uniformly 

 over every part ; but if we soak it in a little water, and then 

 wash the paper with a soft sponge, all the prussian blue is re- 

 moved, except over a space which exceeds the limits of the 

 visible spectrum at both ends. Below the visible red rays the 

 action appears to have extended to the maximum heat spot, 

 so well developed in the experiments of Sir John Herschel, 

 the blue deposit forming a well-defined circular termination, 

 below which are some evidences of a protecting action similar 

 to that exerted by the " extreme red rays " in many instances. 

 Beyond the violet rays the action is extended over the invi- 

 sible space to an extent nearly equal to that of the so-called 

 "visible chemical rays." The action is tolerably uniform 

 throughout the whole extent of this spectrum, but there are 

 evidences of a maximum action in the mean blue ray. I en- 

 close you the spectral image thus formed. 



If we soak one of these negative chromo-cyanotypes in a 

 solution of pure potash or ammonia, the picture is obliterated, 

 but it may be revived by exposure to sunshine, or by the ap- 

 plication of heat; but in both cases the parts which were blue 

 become brown. 



The picture is still more perfectly obliterated if we expose 

 it to the action of the nitrate of mercury and sunshine at the 

 same time ; a sheet of blank white paper apparently remains, 

 but the picture is only clouded, not destroyed. If held before 

 a warm fire, or still better, if a hot iron is applied, a positive 

 image of some intensity replaces the negative one. This pe- 

 culiarity has been noticed in the cyanotypes by Sir John 

 Herschel, but in the instances he has given the picture is im- 

 proved by washing out the mercurial salt previously to the ap- 

 plication of heat, but if this is done in the present instance, the 

 revived picture is exceedingly faint. Such are the interesting 



