When a direct spectrum is thrown on colours darker than itself, it mixes
with them; as the yellow spectrum of the setting sun, thrown on the green
grass, becomes a greener yellow. But when a direct spectrum is thrown on
colours brighter than itself, it becomes instantly changed into the reverse
spectrum, which mixes with those brighter colours. So the yellow spectrum
of the setting sun thrown on the luminous sky becomes blue, and changes
with the colour or brightness of the clouds on which it appears. But the
reverse spectrum mixes with every kind of colour on which it is thrown,
whether brighter than itself or not; thus the reverse spectrum, obtained by
viewing a piece of yellow silk, when thrown on white paper, was a lucid
blue green; when thrown on black Turkey leather, becomes a deep violet. And
the spectrum of blue silk, thrown on white paper, was a light yellow; on
black silk was an obscure orange; and, the blue spectrum, obtained from
orange-coloured silk, thrown on yellow, became a green.

In these cases the retina is thrown into activity or sensation by the
stimulus of external colours, at the same time that it continues the
activity or sensation which forms the spectra; in the same manner as the
prismatic colours, painted on a whirling top, are seen to mix together.
When these colours of external objects are brighter than the direct
spectrum which is thrown upon them, they change it into the reverse
spectrum, like the admission of external light on a direct spectrum, as
explained above. When they are darker than the direct spectrum, they mix
with it, their weaker stimulus being inefficient to induce the reverse
spectrum.

3. _Variation of spectra in respect to number, and figure, and remission._

[Illustration: Fig. 4.]

When we look long and attentively at any object, the eye cannot always be
kept entirely motionless; hence, on inspecting a circular area of red silk
placed on white paper, a lucid crescent or edge is seen to librate on one
side or other of the red circle: for the exterior parts of the retina
sometimes falling on the edge of the central silk, and sometimes on the
white paper, are less fatigued with red light than the central part of the
retina, which is constantly, exposed to it; and therefore, when they fall
on the edge of the red silk, they perceive it more vividly. Afterwards,
when the eye becomes fatigued, a green spectrum in the form of a crescent
is seen to librate on one side or other