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could be done. The eye was therefore caused to approach
the dark focus, no defence, in the first instance, being pro-
vided; but the heat, acting upon the parts surrounding the.
pupil, could not be borne. An aperture was, therefore,
pierced in a plate of metal, and the eye placed behind the
aperture, was caused to approach the point of convergence
of invisible rays. The focus was attained, first by the
pupil and afterward by the retina. Removing the eye, but
permitting the plate of metal to remain, a sheet of platinum-
foil was placed in the position occupied by the retina a mo-
ment before. The platinum became red hot. N o sensible
damage was done to the eye by this experiment; no im-
pression of light was produced; the Optic nerve was not
even conscious of heat.

But the humors of the eye are known to be highly im-
pervious to the invisible calorific rays, and the question
therefore arises, “ Did the radiation in the foregoing experi-
ment reach the retina at all?” The answer is, that the
rays were in part transmitted to the retina, and in part ab-
sorbed by the humors. Experiments on the eye of an ox
showed that the proportion of obscure rays which reached
the retina amounted to 18 per cent. of the total radiation;
while the luminous emission from the electric light amounts
to no more than 10 per cent. of the same total. Were the
purely luminous rays of the electric lamp converged by our
mirror to a focus, there can be no doubt as to the fate of a
retina placed there. Its ruin would be inevitable; and yet
this would be accomplished by an amount of wave-metion
but little more than half of that which the retina bears,
without exciting consciousness, at the focus of invisible

This subject will repay a moment’s further attention.
At a common distance of a foot the visible radiation of the
electric light is 800 times the light of a candle. At the
same distance, the portion of the radiation of the electric

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