ther; or when they are disposed in each other's
vicinity, so as successively to affect the organ of vision.

In a paper on the colours seen in the eye after looking for some time
on luminous objects, published by Dr. Darwin of Shrewsbury in the
Philos. Trans. Vol. 76, it is evidently shown, that we see certain
colours not only with greater ease and distinctness, but with relief
and pleasure, after having for some time inspected other certain
colours; as green after red, or red after green; orange after blue, or
blue after orange; yellow after violet, or violet after yellow; this,
he shows, arises from the ocular spectrum of the colour last viewed
coinciding with the irritation of the colour now under contemplation.

Thus if you make a dot with ink in the centre of a circle of red silk
the size of a letter-wafer, and place it on a sheet of white paper,
and look on it for a minute without moving your eyes; and then gently
turn them on the white paper in its vicinity, or gently close them,
and hold one hand an inch or two before them, to prevent too much
light from passing through the eyelids, a circular spot of pale green
will be seen on the white paper, or in the closed eye; which is called
the ocular spectrum of the red silk, and is formed as Dr. Darwin shows
by the pandiculation or stretching of the fine fibrils, which
constitute the extremities of the optic nerve, in a direction contrary
to that, in which they have been excited by previously looking at a
luminous object, till they become fatigued; like the yawning or
stretching of the larger muscles after acting long in one direction.

If at this time the eye, fatigued by looking long at the centre of the
red silk, be turned on paper previously coloured with pale green; the
circular spot or ocular spectrum will appear of a much darker green;
as now the irritation from the pale green paper coincides with the
pale green spectrum remaining in the eye, and thus excites those
fibres of the retina into stronger action; on this account some
colours are seen more distinctly, and consequently more agreeably
after others; or when placed in the vicinity of others; thus if
orange-coloured letters are painted on a blue ground, they may be read
at as great distance as black on white, perhaps at a greater.

The colours, which are thus more distinct when seen in succession are
called opposite colours by Sir Isaac Newton in his optics, Book I.
Part 2, and may be easily discovered b