aperture of the telescope, but that generally in the morning and in
the evening, when there are more vapours near the Earth, these objects
seem to rise higher, so that the half or more of them will no longer
be visible; and so that they seem lower toward mid-day when these
vapours are dissipated.

Those who consider refraction to occur only in the surfaces which
separate transparent bodies of different nature, would find it
difficult to give a reason for all that I have just related; but
according to our Theory the thing is quite easy. It is known that the
air which surrounds us, besides the particles which are proper to it
and which float in the ethereal matter as has been explained, is full
also of particles of water which are raised by the action of heat; and
it has been ascertained further by some very definite experiments that
as one mounts up higher the density of air diminishes in proportion.
Now whether the particles of water and those of air take part, by
means of the particles of ethereal matter, in the movement which
constitutes light, but have a less prompt recoil than these, or
whether the encounter and hindrance which these particles of air and
water offer to the propagation of movement of the ethereal progress,
retard the progression, it follows that both kinds of particles flying
amidst the ethereal particles, must render the air, from a great
height down to the Earth, gradually less easy for the spreading of the
waves of light.

[Illustration]

Whence the configuration of the waves ought to become nearly such as
this figure represents: namely, if A is a light, or the visible point
of a steeple, the waves which start from it ought to spread more