image of the external area of yellow paper, and could therefore produce
only a blue halo round the yellow spectrum in the center.

If any one should suspect that the scattered rays from the exterior
coloured object do not intermix with the rays from the interior coloured
object, and thus affect the central part of the eye, let him look through
an opake tube, about two feet in length, and an inch in diameter, at a
coloured wall of a room with one eye, and with the other eye naked; and he
will find, that by shutting out the lateral light, the area of the wall
seen through a tube appears as if illuminated by the sunshine, compared
with the other parts of it; from whence arises the advantage of looking
through a dark tube at distant paintings.

Hence we may safely deduce the following rules to determine before-hand the
colours of all spectra. 1. The direct spectrum without any lateral light is
an evanescent representation of its object in the unfatigued eye. 2. With
some lateral light it becomes of a colour combined of the direct spectrum
of the central object, and of the circumjacent objects, in proportion to
their respective quantity and brilliancy. 3. The reverse spectrum without
lateral light is a representation in the fatigued eye of the form of its
objects, with such a colour as would be produced by all the primary
colours, except that of the object. 4. With lateral light the colour is
compounded of the reverse spectrum of the central object, and the direct
spectrum of the circumjacent objects, in proportion to their respective
quantity and brilliancy.

2. _Variation and vivacity of the spectra occasioned by extraneous light._

The reverse spectrum, as has been before explained, is similar to a colour,
formed by a combination of all the primary colours, except that with which
the eye has been fatigued in making the experiment: so the reverse spectrum
of red is such a green as would be produced by a combination of all the
other prismatic colours. Now it must be observed, that this reverse
spectrum of red is therefore the direct spectrum of a combination of all
the other prismatic colours, except the red; whence, on removing the eye
from a piece of red silk to a sheet of white paper, the green spectrum,
which is perceived, may either be called the reverse spectrum of the red
silk, or the direct spectrum of all the rays from the white paper, except
the red; for in truth it is both. Hence we see the reason why it is not