128 
POPULAR SCIENCE REVIEW, 
The quantitative question, in reference to the amounts of silver and 
gold necessarily expended in the operation of photographic printing, has 
received some further elucidation, by experiments lately made in this 
direction by Mr. John Spiller, assistant chemist to the War Department, at 
Greenwich. The practical conclusions established by this gentleman’s 
analytical results are to the effect, that upwards of eighty per cent, of the 
silver originally employed in the process of sensitizing the paper may be 
recovered in the state of precipitated chloride and sulphide, by judiciously 
collecting the various washings and waste solutions, and treating them 
with common salt ; or, in the case of the fixing baths, and other solutions 
containing hyposulphides, with crude sulphide of potassium. These pro- 
ducts being separately reduced to metal, are stated to yield in the aggre- 
gate the large amount of silver indicated, from which the pure nitrate 
may be again prepared, and, at the same time, a certain proportion of 
gold rescued for further employment in the toning process. By adopting 
the suggestions of the author, it appears that of the fifty grains of nitrate 
of silver ordinarily taken up by the full-sized sheet of albumenized paper, 
at least forty grains are recoverable, and that, at the rate of four shillings 
per ounce troy, one pennyworth of silver need only be actually expended 
in the production of a single proof of the largest possible size (seventeen 
by twenty-two inches). 
The most striking improvement in construction which we feel called 
upon to notice is one referring to the camera. In the ordinary form of 
the double-bodied instrument, it has hitherto been customary to make the 
hinder portion slide within the front case, to meet the adjustment for 
focus ; but, by reversing this arrangement (which is now becoming a 
general practice), the lens and smaller front part of the camera are made 
to travel forwards by the action of an endless screw in the base board, 
the ground glass remaining stationary, by which means the operation of 
focusing is more readily accomplished than when, on the old plan, the 
image is constantly approaching or receding from the eye. Another 
advantage gained by this arrangement is, the increased accommodation 
provided for the reception of the plate-holder at the back of the camera, 
and the possibility of taking therein pictures of somewhat larger dimen- 
sions, on account of the interior form of the instrument humouring, instead 
of being opposed to, the direction taken by the rays of light diverging 
from the lens. The large camera, made on this principle by Mr. Dall- 
meyer, and shown in the nave of the International Exhibition, is one 
among many illustrations of this novelty in construction. 
PHYSICS : LIGHT, HEAT, AND ELECTRICITY. 
PAPER, by A. J. Angstrom, has been communicated to the Editor 
of the Philosophical Magazine, “ On the Fraunhofer’s-lines visible 
in the Solar Spectrum.” He repeats the statement, “ that a body in a 
state of glowing heat emits just the same kinds of light and heat which 
it absorbs under the same circumstances ; ” and considers that “ the 
electric spectrum must be viewed as the superposition of two spectra — 
