44 
POPULAR SCIENCE REVIEW. 
comparison with the sensitiveness of ordinary photographic 
paper in the earliest days of the art science, it is very much 
slower than any of the preparations used at the present day 
for obtaining pictures in the camera. With a lens of large 
angular aperture, a brightly lighted landscape might require an 
exposure of an hour. Not so, however, if the light were allowed 
to act upon the sensitive surface through coloured or painted 
glass superimposed on the plate ; or if the solar spectrum were 
allowed to fall upon it ; under these circumstances, an exposure 
of a few minutes would suffice. 
Another process of M. Becquerel, in which galvanic electri- 
city is employed in the preparation of the plates, gives excellent 
results, and is the one by which was produced the spectrum in 
the miniature case referred to as having been presented by him 
to Sir David Brewster. This process was discovered in 1849, or 
about ten years after Sir J. Herschel’s first experiment. He 
had observed that red rays which exercise almost no action 
upon sensitive paper prepared in the dark, acted much more 
rapidly upon the same paper after it had been exposed to light ; 
and that while the paper in the first of these conditions gave 
only a brown or slightly violet colour in the actinic rays, paper 
in the second state, or after the exposure to light, gave variable 
colours, recalling those of the rays which produced them, and 
even developed those colours in the less refracted part of the 
spectrum. Pursuing the subject, he was led to a certain 
method for preparing plates, on which could be received 
coloured impressions, and which, divested of many reasons which 
M. Becquerel communicated to the writer for the adoption and 
discarding of certain lines of procedure, is substantially as 
follows : — An ordinary daguerreotype plate — that is, a plate of 
copper faced with silver — is polished, and the back having 
been protected by varnish, is attached by means of two hooks 
to the positive pole of a voltaic battery. To the negative pole 
is attached a piece of platinum foil, and both are then im- 
mersed in diluted hydrochloric acid, one part of pure acid to 
eight parts of water being a suitable degree of strength. In 
the course of about a minute the plate will have passed through 
several stages of colouration, including grey, yellow, blue, green, 
rose, violet and blue. When of a blackish violet colour the 
action is stopped and the plate is washed, then slightly rubbed 
with cotton wool, and dried. It is now ready for being im- 
pressed with the colours of the spectrum. If a plate so pre- 
pared be heated before exposure, it acquires new properties. It 
changes to a red colour, which is the state most suitable for 
receiving all the colours. Exposure to the sun under a sheet 
of paper impregnated with sulphate of quinine confers the 
same properties as the exposure to heat. 
