that no sensible impulse could be observed, but what might well be ascribed
to the ascent of heated air.

Whence it is reasonable to conclude, that the light of the day must be much
too weak in its dilute state to make any mechanical impression on so
tenacious a substance as the retina of the eye.--Add to this, that as the
retina is nearly transparent, it could therefore make less resistance to
the mechanical impulse of light; which, according, to the observations
related by Mr. Melvil in the Edinburgh Literary Essays, only communicates
heat, and should therefore only communicate momentum, where it is
obstructed, reflected, or refracted.--From whence also may be collected the
final cause of this degree of transparency of the retina, viz. left by the
focus of stronger lights, heat and pain should have been produced in the
retina, instead of that stimulus which excites it into animal motion.

2. On looking long on an area of scarlet silk of about an inch in diameter
laid on white paper, as in Plate I. the scarlet colour becomes fainter,
till at length it entirely vanishes, though the eye is kept uniformly and
steadily upon it. Now if the change or motion of the retina was a
mechanical impression, or a chemical tinge of coloured light, the
perception would every minute become stronger and stronger,--whereas in
this experiment it becomes every instant weaker and weaker. The same
circumstance obtains in the continued application of sound, or of sapid
bodies, or of odorous ones, or of tangible ones, to their adapted organs of
sense.

[Illustration: Plate II.]

Thus when a circular coin, as a shilling, is pressed on the palm of the
hand, the sense of touch is mechanically compressed; but it is the stimulus