as the source of light and the distinction of day from night, yet, in transmitted light, it supplied a motor, which was required for the completion of his own theory of sensation through the agency of a medium acted upon by impulsion.
Note 4, p. 95. Now that which is without colour, &c.]
The diaphaneity, that is, when passive, is receptive of
colour and made active, just as the air, when quite still,
is more readily set in motion and made sonorous by per-
cussion; and this leads, amid some confusion of thought,
to the consideration of those luminous appearances (ignes
fatui) which are visible only in the dark, by their colour.
'The precise nature of these appearances is still only
conjectural, notwithstanding the advance of chymistry;
but they are supposed to be due to phosphyretted
hydrogen eliminated, under favouring circumstances, from
decaying animal and vegetable matter, and ignited by
contact with the atmosphere."
Note 5, p. 95. Therefore, witliout light colour is not visible.] Colour, that is, by imparting motion to the diaphaneity, renders it, from being potential and dark, actual and visible, that is, light; and thus, as without light there is no colour, so without colour there is no light; and this lends support to the opinion, that the air, as being a diaphanous medium, is essential to sight. Aristotle had indeed maintained, in opposition to Empedocles[1] and others, that vision is not caused by the
- ↑ De Sensu et Sens. II. 15, 16.
