m the eye, the spectrum will appear magnified in
proportion to the distance.

[Illustration: Fig. 5.]

I was surprised, and agreeably amused, with the following experiment. I
covered a paper about four inches square with yellow, and with a pen filled
with a blue colour wrote upon the middle of it the word BANKS in capitals,
as in Fig. 5, and sitting with my back to the sun, fixed my eyes for a
minute exactly on the center of the letter N in the middle of the word;
after closing my eyes, and shading them somewhat with my hand, the word was
distinctly seen in the spectrum in yellow letters on a blue field; and
then, on opening my eyes on a yellowish wall at twenty feet distance, the
magnified name of BANKS appeared written on the wall in golden characters.

_Conclusion._

It was observed by the learned M. Sauvage (Nosol. Method. Cl. VIII. Ord.
i.) that the pulsations of the optic artery might be perceived by looking
attentively on a white wall well illuminated. A kind of net-work, darker
than the other parts of the wall, appears and vanishes alternately with
every pulsation. This change of the colour of the wall he well ascribes to
the compression of the retina by the diastole of the artery. The various
colours produced in the eye by the pressure of the finger, or by a stroke
on it, as mentioned by Sir Isaac Newton, seem likewise to originate from
the unequal pressure on various parts of the retina. Now as Sir Isaac
Newton has shewn, that all the different colours are reflected or
transmitted by the laminae of soap bubbles, or of air, according to their
different thickness or thinness, is it not probable, that the effect of the
activity of the retina may be to alter its thickness or thinness, so as
better to adapt it to reflect or transmit the colours which stimulate it
into action? May not muscular fibres exist in the retina for this purpose,
which may be less minute than the locomotive muscles of microscopic
animals? May not these muscular actions of the retina constitute the
sensation of light and colours; and the voluntary repetitions of them, when
the object is withdrawn, constitute our memory of them? And lastly, may not
the laws of the sensations of light, here investigated, be applicable to
all our other senses, and much contribute to elucidate many phenomena of
animal bodies both in their healthy and diseased state; and thus render
this investigation well worthy the attention of the physician, the
metaphysicia