The hypothesis of vibrations, which was much extend ed and illustrated by Hartley, supposes that when an im pression is made upon the extremity of a nerve, a pecu liar kind of vibration is excited among the particles of the nervous matter ; this is transmitted to the brain, and there produces a corresponding vibration, which is the imme diate cause of perception. This hypothesis is more easy to comprehend than that of the animal spirits, and, in some respects, accords better with the nature of the ac tion that is excited ; but it is, like the other, wholly devoid of direct evidence, and appears inappropriate to the nature of the organ. It accordingly met with but few advocates when it was first proposed, and is now almost totally ne glected, so that we are reduced to the necessity of con fessing our ignorance of the mode in which sensations are conveyed to the brain, and are there rendered per ceptible to the mind.* With respect to the eye in par ticular, although we are well acquainted with the physi cal properties of light, and with the manner in which the rays enter the eye, and form a focus upon the retina, yet w e are quite unable to explain how this picture operates in producing vision, how it acts upon the retina, how the retina communicates with the nerve, and how this affects t he brain.
But although we arc ignorant of the nature of the pro cess by which sensation is produced, there are some points respecting it which require to be noticed, and which par ticularly apply to the sense of sight, as we have the spe cific cause of this sense entirely under our command, and can accurately observe the direct effects produced by it. When an impression has been made upon the extremity of a nerve, the sensation remains for some time after the cause is removed, as is often observed with respect to the eve, where, after the application of a bright light, the im pression continues for some time after the light is remov ed. If a burning body be rapidly whirled round, it will produce the appearance of a compIete circle of fire ; or if the seven prismatic colours be painted upon a card, and the card be made to spin upon its centre,no individual colour w ill be seen, but the eye will merely receive the impres sion of whiteness, arising from the combined impression of the whole. These effects depend upon the principle, that the eye retains the impression of the in each particular part of the circle, until it arrives again at the same part, and the different impressions are thus all con founded together.
Analogous to these phenomena, although acting in a somewhat different way, are those which have been term ed ocular spectra. These appearances were noticed by Buffon, who styled them accidental colours, and they have been since more minutely described by Darwin. If the eye
he steadily directed to a bright spot on a dark ground, and then turned aside, an image of the spot will be perceived ; but the spot will now appear dark and the ground white, and the reverse will take place, if we view a dark spot on a bright ground. This alternation takes place likewise between diflerent colours, as well as between different de grees of light. If we look at a blue object, the eye ac quires a yellow spectrum; if at a yellow object, a blue spec trum ; green produces a red, and red a green spectrum ; in short, every colour has its appropriate spectral colour, which is inseparably connected with it, and is produced, with more or less intensity, according to the brilliancy of the object and the pecular condition of the vision. It is probable that the harmony of colouring, as it exists in paintings and in the arrangement of furniture and drapery, depends very much upon this affection of the eye, although we are guided in these cases by experience, without any conception of the principle on which it rests. Sec Acct DENTAL COLOURS.
There is reason to believe, that the formation of these spectra, aided by mental impressions of various kinds, have frequently given rise to the belief of supernatural appearances. There are certain diseased states of the system generally, and of the nerves in particular, when the retina is peculiarly disposed to retain these images, so that even after the object has been removed for some time, if the light be withdrawn, the spectrum will still be visible. The surprise which such appearances must oc casion to those totally ignorant of their nature, the terror which is often associated with darkness, and other con current causes, may be conceived to operate powerfully upon weak minds, perhaps farther debilitated by fatigue or disease.
The cause of this peculiar state of vision is obscure. It has been referred to a property of the nervous system, by which a part is unable to persevere in the same kind of ac tion beyond a limited petiod, producing, what has been styled, exhaustion. The term exhaustion must be regard ed as metaphorical ; it was derived from the hypothesis of animal spirits, upon the principle that there is a limited supply of these spirits in the nerve, which, by a too long continuance of the action, becomes expended, and that it is necessary for some time to elapse before the nerve can again acquire the due quantity of this agent to renew its functions. But it is scarcely necessary to observe that this reasoning presupposes the existence of the animal spirits, an hypothesis, which is itself without foundation.