FIRST BOOK ON LIGHT AND SHADE.

On the nature of light (130. 131).

The reason by which we know that a light radiates from a single
centre is this: We plainly see that a large light is often much
broader than some small object which nevertheless--and although the
rays [of the large light] are much more than twice the extent [of
the small body]--always has its shadow cast on the nearest surface
very visibly. Let _c f_ be a broad light and _n_ be the object in
front of it, casting a shadow on the plane, and let _a b_ be the
plane. It is clear that it is not the broad light that will cast the
shadow _n_ on the plane, but that the light has within it a centre
is shown by this experiment. The shadow falls on the plane as is
shown at _m o t r_.

[Footnote 13: In the original MS. no explanatory text is placed
after this title-line; but a space is left for it and the text
beginning at line 15 comes next.] Why, to two [eyes] or in front of
two eyes do 3 objects appear as two?

Why, when you estimate the direction of an object with two sights
the nearer appears confused. I say that the eye projects an infinite
number of lines which mingle or join those reaching it which come to
it from the object looked at. And it is only the central and
sensible line that can discern and discriminate colours and objects;
all the others are false and illusory. And if you place 2 objects at
half an arm's length apart if the nearer of the two is close to the
eye its form will remain far more confused than that of the second;
the reason is that the first is overcome by a greater number of
false lines than the second and so is rendered vague.

Light acts in the same manner, for in the effects of its lines
(=rays), and particularly in perspective, it much resembles the eye;
and its central rays are what cast the true shadow. When the object
in front of it is too quickly overcome with dim rays it will cast a
broad and disproportionate shadow, ill defined; but when the object
which is to cast the shadow and cuts off the rays near to the place
where the shadow falls, then the shadow is distinct; and the more so
in proportion as the light is far off, because at a long distance
the central ray is less overcome by false rays; because the lines
from the eye and the solar and other luminous rays passing through
the atmosphere are obliged to travel in straight lines. Unless they
are deflected by a denser or rarer air, when they will be bent at
some point, but so long as the air is free from grossness or
moisture they will preserve their direct course, always carrying the
image of the object that intercepts them back to their point of
origin. And if this is the eye, the intercepting object will be seen
by its colour, as well as by form and size. But if the intercepting
plane has in it some small perforation opening into a darker
chamber--not darker in colour, but by absence of light--you will see
the rays enter through this hole and transmitting to the plane
beyond all the details of the object they proceed from both as to
colour and form; only every thing will be upside down. But the size
[of the image] where the lines are reconstructed will be in
proportion to the relative distance of the aperture from the plane
on which the lines fall [on one hand] and from their origin [on the
other]. There they intersect and form 2 pyramids with their point
meeting [a common apex] and their bases opposite. Let _a b_ be the
point of origin of the lines, _d e_ the first plane, and _c_ the
aperture with the intersection of the lines; _f g_ is the inner
plane. You will find that _a_ falls upon the inner plane below at
_g_, and _b_ which is below will go up to the spot _f_; it will be
quite evident to experimenters that every luminous body has in
itself a core or centre, from which and to which all the lines
radiate which are sent forth by the surface of the luminous body and
reflected back to it; or which, having been thrown out and not
intercepted, are dispersed in the air.