46 
POPULAR SCIENCE REVIEW. 
surrounding subjects green and yellow. The order of the action 
of the spectral rays was quite the reverse of what it is in 
photography of the usual kind, for the red cloak was strongly 
impressed in a good bright red, while the blue had scarcely 
begun to undergo a change ; the green and yellow, too, were 
inferior to the red. After a continuation of the exposure for 
another quarter of an hour, a further examination showed 
a great increase in the detail of the refrangible rays, while the 
red had not become in the least degree deteriorated by the 
increased exposure. A more prolonged exposure brought out 
all the colours. The prints were fixed by immersion successively 
in water, acidulated with chromic acid, a weak solution of 
nitrate of lead and distilled water. At the time of these ex- 
periments we were not favourably impressed with the per- 
fection of the fixing; nevertheless, the heliochromic prints 
thus produced remained in a tolerable state of preservation for 
about eight weeks, after which time they were presented to a 
friend. 
By modifying Poitevin’s formula, M. de S. Florent has greatly 
increased the sensitiveness of the paper as well as the perfec- 
tion of the fixing. By immersing the subchlorised paper in a 
bath of a very weak solution of nitrate of mercury, and after 
partially drying with blotting paper, the sheets of paper are 
rendered so sensitive as to receive from a superimposed coloured 
cliche a vigorous impression in a quarter of a minute. By 
adding to the bath just mentioned a mixture composed of 
two parts of a saturated solution of bichromate of potash, two 
parts of sulphuric acid, and one part of chlorate of potash, the 
colours are more vivid without any diminution of the sensitive- 
ness. Fixing is effected in ammonia diluted with alcohol 
in the proportion of one part of the former to twenty of the 
latter, followed by immersion in an alkaline chloride solution. 
Curiously enough, an increased degree of rapidity is found to 
be obtained if the subchloride of silver formed on the paper is 
prepared by exposure to light under a violet glass. The end 
of the year 1873 finds the problem of photography in natural 
colours far nearer a satisfactory solution than did the beginning 
of the year. It is to be regretted that the number of experi- 
mentalists in this field of research is exceedingly limited. 
Pending the bringing to a practical issue the production of 
natural polychromatic photographs, attempts, some of them 
tolerably successful, have been made to combine colour and 
photography by means more or less mechanical. A few years 
ago, Mr. Kobert Howe Ashton obtained a patent for a method 
of applying colour to photographs produced by the Woodbury 
process — a method which, although it has not yet been com- 
mercially developed to the extent its merits demand, is yet 
