42 
POPULAR SCIENCE REVIEW. 
this possibility has never been altogether absent from the 
minds of chemists since the wonders of what photography 
actually can accomplish were first revealed to view. In spite 
of many failures which have come of attempts hitherto made to 
realise this hope, it is by no means yet surrendered by all. At 
the same time, it has seemed sufficiently near impossibility of 
realization to induce many to turn aside and seek the fulfilment 
of it by other than photographic means. The hand of the 
painter or lithographer has been called in to eke out the short- 
comings of the chemical rays ; and no insignificant success has 
been attained in the effort to combine science and art in this 
field. There are thus two distinct lines of enquiry, which we 
shall endeavour to trace separately. 
It is not a little singular that while the present high state 
to which photography has already been brought is owing mainly 
to the inventive faculty and prolific process-engendering 
research of Englishmen, it is to a few French philosophers 
that we are indebted for nearly everything that has yet been 
effected in the direction of photo-polychromy, both in the 
natural or chemical, and in the mechanical departments. 
So long ago as 1839 the late Sir John Herschel noticed the 
interesting fact that paper which had been rendered sensitive 
by chloride of silver, aud had been exposed to light until it had 
darkened, was then in a condition to reproduce various colours 
when again exposed to the action of light under strongly- 
coloured pieces of glass. Although Sir John Herschel did not 
continue this line of investigation, he says that he saw enough 
from his first experiments to warrant him in arriving at the 
conclusion that photography in natural colours might reason- 
ably be expected to be brought within the range of possible 
accomplishment. It is an act of justice to our distinguished 
countryman to place on record the fact that the most successful 
pictures in natural colours that have been produced in times of 
more recent date, and by other experimentalists, have been 
obtained by means of processes based on his discovery of the 
capability of sub-chloride of silver to receive coloured impres- 
sions. G-uided by the directions of M. Poitevin, we have taken 
many pictures in natural colours by a method discovered by 
the latter gentleman, but under which there ran as a sub- 
stratum the previous discovery of Herschel which, if crude, 
was at least suggestive. 
It may be observed here, that a valuable suggestion to ex- 
perimentalists in heliochromy is to be found in the fact that 
ordinary writing-paper that has been immersed in, or floated 
upon, a very weak solution of common salt in water, and when 
dry impregnated by being brushed over with ammonio-nitrate 
of silver, will show several colours with a degree of distinct- 
