aced 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 humours of the eye the albumen of the humours might
coagulate along the line of the rays. If, on the contrary, no such
high absorption 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 could be done. The eye was
therefore caused to approach the dark focus, no defence, in the first
instance, being provided; 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 afterwards 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
moment before. The platinum became red-hot. No sensible damage was
done to the eye by this experiment; no impression of light was
produced; the optic nerve was not even conscious of heat.

But the humours of the eye are known to be highly impervious to the
invisible calorific rays, and the question therefore arises, 'Did the
radiation in the foregoing experiment reach the retina at all?' The
answer is, that the rays were in part transmitted to the retina, and
in part absorbed by the humours. 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-motion but little more
than half of that which the retina, without exciting consciousness,
bears at the focus of invisible rays.

This subject will repay a moment