A PHOTOGRAPH IN NATURAL COLOURS. 
43 
ness sufficient, at any rate, to admit of their being easily recog- 
nised. Ammonio-nitrate of silver is in reality a solution of 
oxide of silver in nitrate of ammonia, and is made by adding 
to a solution of nitrate of silver of any degree of strength — 
usually one of sixty grains of the argentic salt to the ounce of 
water — strong ammonia by one or two drops at a time. At 
first, a dense precipitate of oxide of silver is formed, but by 
continuing the addition of the ammonia it is re-dissolved, and 
the liquid becomes clear again. There are other methods of 
preparing paper on which the colours of- the spectrum may be 
obtained ; for example, by first washing the surface of the paper 
with nitrate of silver, and then with fluate of soda ; or by 
means of chloride of barium and nitrate of silver, with a pre- 
vious wash of a dilute solution of iodine. Experiments with 
these substances will prove suggestive, in addition to the in- 
terest that will attach to the experiments per se. 
There are three definite methods of producing, by purely 
chemical means, photographs in natural colours, and these 
differ entirely from each other. 
The writer of the present article had in his possession 
during the summer of 1863, a miniature case, containing a 
silver plate on which was impressed the spectrum in all the 
colours of nature. He obtained it from the late Sir David 
Brewster, whose property it was, for the purpose of exhibiting 
it to H.R.H. the Duke of Edinburgh (then Prince Alfred) and 
Lord Brougham. The colours were decided, although not 
brilliant ; and a special point of interest lay in the incident of 
this picture having been exposed to solar light for nearly a 
fortnight, without any signs of fading being discovered. The 
photograph in question was produced by M. Becquerel, who, 
more than any other, has made heliochromy the subject of 
definite and protracted investigation. These investigations have 
resulted in the discovery of two processes, by which colours 
can be obtained. These we shall describe in detail. 
Bichloride of copper and sulphate of soda are formed by 
double decomposition, arising from the mixture of one ounce 
of powdered sulphate of copper, two ounces of powdered 
chloride of sodium, and five ounces of water. These are mixed 
in a test-glass. Of this solution three ounces are mixed with 
a like quantity of a saturated solution of common salt, and 
eighteen ounces of water ; and this forms the sensitising bath 
for the silver plates. A metallic plate faced with silver, bear- 
ing a high polish, and made very clean, is immersed in this 
solution, and is rapidly coated with a violet-coloured sub- 
chloride, when it is withdrawn, washed, and dried. This is all 
the preparation required for fitting it to receive the colours of 
the spectrum. It is not very sensitive, for although it bears 
