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POPULAR SCIENCE REVIEW. 
exhaustive and, as it seems to us, conclusive series of experiments published 
in the British Journal of Photography, has proved that pure isolated iodide 
of silver is sensitive to light, and capable, by development, of giving a 
bold vigorous image. These experiments consisted in precipitating on a 
perfectly clean glass plate a thin film of pure metallic silver. This film was 
converted entirely into iodide of silver by long immersion in tincture of 
iodine. Every trace of the free iodine having been removed by continuous 
washing, the plate was placed beneath a negative and exposed to daylight for 
some few seconds. A forcible image, contrary to the generally received con- 
clusion, was then developed, and this in the absence of any possible trace of 
a free soluble salt of silver. We have therefore another truth established as 
the basis of. further experiments by the praiseworthy energy and perseverance 
of a very able experimentalist. 
PHYSICS. 
The Spectrum of Comet I, 1866, has been ably investigated by Mr. W. 
Huggins, F.R.S., and described in a paper read by this gentleman before 
the Eoyal Society. The appearance of the comet in the telescope was that of 
an oval nebulous mass, surrounding a very minute and not very bright 
nucleus. The length of the slit of the spectrum apparatus was greater 
than the diameter of the telescopic image of the comet. The appearance pre- 
sented in the instrument when the centre of the comet was brought nearly 
upon the middle of the slit was that of a broad continuous spectrum fading 
away gradually at both edges. These fainter parts of the spectrum cor- 
responded to the more diffused marginal portions of the comet. Nearly in 
the middle of this broad and faint spectrum, and in a position about mid-way 
between b and F of the solar spectrum, a bright point was seen. The absence 
of breadth of this bright point in a direction at right angles to that of the 
dispersion showed that this monochromatic light was emitted from an object 
possessing no sensible magnitude in the telescope. This observation, Mr. 
Huggins concludes, gives to us the information that the light of the coma of 
this comet is different from that of the minute nucleus. The nucleus is self- 
luminous ; and the matter of which it consists is in the state of ignited gas. 
As it cannot be supposed that the coma consists of incandescent solid matter, 
the continuous spectrum of its light probably indicates that it shines by re- 
flected solar light. Since the spectrum of the light of the coma is unlike that 
of the light of the nucleus, it is evident that the nucleus is not the source of 
the light by which the coma is rendered visible to us. Hence, Mr. Huggins 
concludes that the coma of this comet reflects light received from without, 
and the only available source of light is the sun. — Vide Proceedings of Royal 
Society, No. 80, 1866. 
The Spectrum of TempeVs Comet. — In a late number of the Comptes Rendus, 
a letter was published on this subject from Signor Secchi to M. Elie de Beau- 
mont. The Italian physicist has found that the spectrum produced by the 
light of Tempel’s Comet consists of only three bands. One, which cor- 
