Many and important are the inferences which may be deduced
from the foregoing experiment, among which are the following :
1. That light flows in a right line.
2. That a luminous point may be feen from all thofe places to
which a ftrait line can be drawn from the point, without meeting
with any obftacle; and confequently
3- That a luminous point, by fome unknown power, lends
forth rays o f light m all dire&ions, and is the center o f a fphere
o f light, which extends indefinitely on all lides ; and if we conceive
fome o f thefe rays to be intercepted by a plane, then is the
luminous point the fummit o f a pyramid, whofe body is formed '
by the rays, and it’s bafe by the intercepting plane.— The
image o f the furface o f an objeft, which is painted on the wall, is
alfo the bafe o f a pyramid o f light, the apex of which is the
hole; the rays which form this pyramid, by croffing at the hole,
form another, fimilar and oppofite to this, o f which the hole is
alfo the fummit, and the furface of the objefl the bafe.
4- That an objeQ: is vifible, becaufe all it’s points are radiant
points.
5. That the particles o f light are indefinitely fmall ■ for the
rays, which proceed from the points o f all the objefts oppofite to
the hole, pafs through it, though extremely fmall, without em-
harrafling or confounding each other.
6. That
6. That every ray o f light carries with it the image o f the
object from which it was emitted.
The nature o f vifion in the eye, may be imperfeffly illuftrated
by the experiment o f the darkened room ; the pupil of the eye
being confidered as the hole through which the rays o f light pafs,
and crofs each other, to paint on the retina, at the bottom o f the
eye, the inverted images o f all thofe objefts which are expofed to
the fight, fo that the diameter o f the images of the fame objeft
are greater, in proportion to the angles formed at the pupil, by
the crofling rays which proceed from the extremities of the ob-
je£i; that is, the diameter of the image is greater, in proportion
as the diftance is lefs ; or, in other words, the apparent magnitude
o f an objeft is in fome degree meafured by the angle under
which it is feen, and this angle increafes or diminifhes, according
as the objefl is nearer to, or further from the e y e ; and confequently,
the lefs the diftance is between the eye and the objeft,
the larger the latter will appear.
From hence it follows, that the apparent diameter o f an objeft
feen by the naked eye, may be magnified in any proportion we
pleafe ; for as the apparent diameter is increafed, in proportion
as the diftance from the eye is leffened, we have only to lefien
the diftance of the objeft from the eye, in order to increafe the
apparent diameter thereof.* Thus, fuppofe there is an objecb,
A B , Plate I. Fig. l, which to an eve at E fubtends or appears
under the angle A E B , we may magnify the apparent diameter
in what proportion we pleafe, by bringing our eye nearer to it.
If, for inftance, we would magnify it in the proportion o f F G
to
* Rutherforth’s Syftcm of Natural Philofophy, p. 330.