The intersections of the images as they enter the pupil do not
mingle in confusion in the space where that intersection unites
them; as is evident, since, if the rays of the sun pass through two
panes of glass in close contact, of which one is blue and the other
yellow, the rays, in penetrating them, do not become blue or yellow
but a beautiful green. And the same thing would happen in the eye,
if the images which were yellow or green should mingle where they
[meet and] intersect as they enter the pupil. As this does not
happen such a mingling does not exist.
OF THE NATURE OF THE RAYS COMPOSED OF THE IMAGES OF OBJECTS, AND OF
THEIR INTERSECTIONS.
The directness of the rays which transmit the forms and colours of
the bodies whence they proceed does not tinge the air nor can they
affect each other by contact where they intersect. They affect only
the spot where they vanish and cease to exist, because that spot
faces and is faced by the original source of these rays, and no
other object, which surrounds that original source can be seen by
the eye where these rays are cut off and destroyed, leaving there
the spoil they have conveyed to it. And this is proved by the 4th
[proposition], on the colour of bodies, which says: The surface of
every opaque body is affected by the colour of surrounding objects;
hence we may conclude that the spot which, by means of the rays
which convey the image, faces--and is faced by the cause of the
image, assumes the colour of that object.
On the colours of derived shadows (275. 276).
275.
ANY SHADOW CAST BY AN OPAQUE BODY SMALLER THAN THE LIGHT CAUSING THE
SHADOW WILL THROW A DERIVED SHADOW WHICH IS TINGED BY THE COLOUR OF
THE LIGHT.
Let _n_ be the source of the shadow _e f_; it will assume its hue.
Let _o_ be the source of _h e_ which will in the same way be tinged
by its hue and so also the colour of _v h_ will be affected by _p_
which causes it; and the shadow of the triangle _z k y_ will be
affected by the colour of _q_, because it is produced by it. [7] In
proportion as _c d_ goes into _a d_, will _n r s_ be darker than
_m_; and the rest of the space will be shadowless [11]. _f g_ is
the highest light, because here the whole light of the window _a d_
falls; and thus on the opaque body _m e_ is in equally high light;
_z k y_ is a triangle which includes the deepest shadow, because the
light _a d_ cannot reach any part of it. _x h_ is the 2nd grade of
shadow, because it receives only 1/3 of the light from the window,
that is _c d_. The third grade of shadow is _h e_, where two thirds
of the light from the window is visible. The last grade of shadow is
_b d e f_, because the highest grade of light from the window falls
at _f_.
[Footnote: The diagram Pl. III, No. 1 belongs to this chapter as
well as the text given in No. 148. Lines 7-11 (compare lines 8-12 of
No. 148) which are written within the diagram, evidently apply to
both sections and have therefore been inserted in both.]
276.