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9. Deadness of the Optic Mme t0 the Calorific Rays.

The layer of iodine used in the foregoing experiments
intercepted the light of the noonday sun. N o trace of
light from the electric lamp was visible in the darkest
room, even when a white screen was placed at the focus of
the mirror employed to concentrate the light. It was
thought, however, that if the retina itself were brought
into the focus the sensation of light might be experienced.
The danger of this experiment was twofold. If the dark
rays were absorbed in a high degree by the humors of the
eye, the albumen of the humors might coagulate along the
line of the rays. If, on the contrary, no such high ab-
sorption took place, the rays might reach the retina with a
force sufficient to destroy it. To test the likelihood of these
results, experiments were made on water and on a solution of
alum, and they showed it to be very improbable that in the
brief time requisite for an experiment any serious damage

specimens of blue glass, the platinum-foil glowed with a pink or purplish
light. The effect was not subjective, and considerations of obvious in-
terest are suggested by it. Different kinds of black glass differ notably
as to their power of transmitting radiant heat. In thin plates some de-
scriptions tint the sun with a greenish hue: others make it appear a
glowing red without any trace of green. The latter are far more diather-
mic than the former. In fact, carbon when perfectly dissolved, and in-
corporated with a good White glass, is highly tranSparent to the calorific
rays, and by employing it as an absorbent, the phenomena of “ calores-
cence” may be obtained, though in a less striking form than with the
iodine. The black glass chosen for thermometers, and intended to ab-
sorb completely the solar heat, may entirely fail in this object, if the
glass in which the carbon is incorporated be colorless. To render the
bulb of a thermometer a perfect absorbent, the glass ought in the first
instance to be green. Soon after the discovery of fluorescence the late
Dr. William Allen Miller pointed to the lime-light as an illustration of
exalted refrangibility. Direct experiments have since entirely confirmed
the view expressed at page 210 of his work on “Chemistry,” published
in 1855.

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