176 Sir J. F. W. Herschel on the Action of the Rays 
solution (in the shade) a most intense blue spectrum is de- 
veloped over the whole of the more refrangible region, in the 
interior of which the blue colour appears to have been, as it were, 
eaten away, leaving a white oval, as in the specimen annexed ; 
precisely the same phzenomenon, in short, as would have been 
produced under the spectrum had the two liquids acted in 
conjunction. And this white portion comports itself under 
the influence of water or air, just as it would have done had 
it been produced under such joint action; i. e. it gradually 
turns blue till it is no longer distinguishable from the rest of 
the spectrum. It is also blued by ammonia, just as the posi- 
tive paper of Art. 210, after bleaching, would be, &c. In 
short, it is evident that we have succeeded in separating the 
final action described in that article into two distinct steps or 
stages, the photographic influence being confined to the first, 
and the ferrosesquicyanate acting as a mere precipitant on the 
nascent compounds resulting from that influence. 
212. In order to ascertain whether any portion of the iron 
in the double ammoniacal salt employed had really undergone 
deoxidation, and become reduced to the state of protoxide as 
supposed, I had recourse to a solution of gold, exactly neu- 
tralized by carbonate of soda. The proto-salts of iron, as is 
well known to chemists, precipitate gold in the metallic state. 
The effect proved exceedingly striking, issuing in a process 
no wise inferior in the almost magical beauty of its effect to 
the calotype process of Mr. Talbot, which in some respects 
it nearly resembles, with this advantage, as a matter of experi- 
mental exhibition, that the disclosure of the dormant image 
does not require to be performed in the dark, being not inter- 
fered with by moderate daylight. As the experiment will 
probably be repeated by others, I shall here describe it ad 
initio. Paper is to be washed with a moderately concentrated 
solution of ammonio-citrate of iron, and dried. The strength 
of the solution should be such as to dry into a good yellow 
colour, not at all brown. In this state it is ready to receive 
a photographic image, which may be impressed on it either 
from nature in the camera-obscura, or from an engraving on 
a frame in sunshine. The image so impressed, however, is 
very faint, and sometimes hardly perceptible. The moment 
it is removed from the frame or camera, it must be washed 
over with a neutral solution of gold of such strength as to 
have about the colour of sherry wine. Instantly the picture 
appears, not indeed at once of its full intensity, but darkening 
with great rapidity up to a certain point, depending on the 
strength of the solutions used, &c. At this point nothing can 
surpass the sharpness and perfection of detail of the resulting 
