Upon this chemistry of light we may easily account for the variety of colours so beautifully displayed in vegetal nature, and principally in flowers, which acquire their colours as they expand, and undergo all the relative changes of hue and tint in their progress and decay, which the immediate combination of these chemical agents may be made to produce in the elaboratory of the chemist.

Vision itself, and its various phenomena, physically explained, are to be regarded as dependent on the same subtile chemistry; for nature is ever simple and uniform. If hence the eye, by the agency of the retina and optic nerve, have equal affinity for both elements of light, it will at once discern their intensity or power in light and shade, and also their inequalities in colours; and this will explain why the eye is incapacitated to see when passing suddenly from darkness to light, or from light to darkness; while unequal affinities of the organ may explain the various defects and disorders of vision with regard to colours,* and the use and abuse of coloured spectacles as remedies thereof. The eye may be regarded as an electrical machine that accumulates the principles of light, and separates and recombines them with the like principles of external light reflected from the objects of vision.

* Exleya "Optics," Ph. 14, p. 23. \ See Note F.

This doctrine of the secretion of the elements of light by the organs of vision, and their exhaustion by external objects, which coincides with our preceding chemical theory of light and colours, and conjointly therewith appears to us to explain all the phenomena of vision more solidly than any other hitherto advanced, is strengthened by an effect of sight we have recently experienced with deep attention, while subject to inflammation of the eyes, attended by the constant appearance of a vibrating internal light of extreme vividness, in the form of a colourless rose of great beauty, the leaves of which whirled into each other centripetally and continually, night and day, during many weeks; with some variation of hue and intensity from a cool bright to a warm dull colour.

• See Note G.

Under a similar affection of the ear we have experienced a continuation of internal musical sounds, which are, perhaps, attributable to similar agency; as may also corresponding affections of the other senses. Now, the true physical cause of sensation in either of its organs being once established, we shall have an unfailing clue to the cause of all sensation, and to the true physical foundation of their respective sciences.*

As the principles of external light exhaust the principles of light in the eye, it is worthy of our attention that the action of direct light, or strongly reflected light, upon the organ, is destructive of vision, temporarily or for ever; and as the angles of reflection and incidence in light are co-equal, it is obvious that we see objects to most advantage for vision and the organ, by avoiding the angle of incidence, or by a position half turned from the light. It is true that in cases of extreme action of light on the eye, instinct and nature lead or compel us to these observances, but we are too inattentive to them in other cases, as in reading, writing, drawing, etc,; and thereby a contrary habit becomes established, tilt injured vision or blindness ensue.

* See our " Analogical Philosophy," vol. ii. pp. 83, 89, and 97, where we have treated of this subject as a branch of universal science, physically and aerthetically.

This theory of vision affords also a solution of the curious phenomena of ocular spectra, in which the eye discerns those adventitious or accidental colours, first especially treated of by Dr. Jurin,* and subsequently by Buffon and others, which have no apparent cause out of the organ itself; for the equal affinity of the eye for the two principles of light and colours, is of course destroyed by the action of a colour in which either of them predominates, the predominant principle neutralizing or exhausting its opposite principle in the organ, while its other principle therein continues free, or accumulates during the act of intent vision; and, therefore, the organ decomposes, by a due election, the light of other objects to which it is impelled to wander, till the balance of principles in the organ itself is restored.† Accordingly the adventitious colour, or spectrum, occasioned by an object intently viewed is always of its opposite hue, or that compensatory or harmonic colour which restores the equilibrium or neutrality of light and vision; and that the eye does this by secreting these principles of light, and retaining them in a latent state, is evinced by the light and dark circles which arise when the side of the closed eye is pressed, by the spark elicited upon the puncturing of the retina in the operation of couching, and also by the extreme sensibility of the eye to light after being long secluded from it.

"In Smith's "Optics." Kirclier also has noticed an effect of this kind in his " Ars Magna Lucis et Umbra:," p. 118; as had Aristotle also among the antietita.

† See Note H.