pression of the whole. These effects depend upon the principle, that the eye retains the impression of the object in each particular part of the circle, until it arrives again at the same point, so that the different or successive impressions are all blended together.

There is another very curious series of phenomena, which are somewhat analogous to the above, as far at least as they depend upon the permanency of the effect after the exciting cause is removed. They were first minutely described by Buffon, who named them accidental colours,* they were afterwards successively examined by Schaffer, Epinus, and Darwin, and are now known by the name of ocular spectra. If the eye be steadily directed, for some time, to a white spot upon a dark ground, and be then turned aside, we shall perceive a well defined image of the spot, but the effect will be reversed; the spot will now appear dark and the ground white, and the opposite effect will be produced if we view a dark spot upon a white ground. The same kind of alternation takes place between different colours as between different degrees of light; if, for example, we look at a blue object, the eye acquires a yellow spectrum, while a yellow object produces a blue spectrum. In the same manner red and green alternate with each other, and in short every colour has its appropriate spectral colour, the sensation of which is always produced in the eye, when the primary colour has made a sufficiently strong impres sion upon it. It may be presumed, that a considerable share of what is termed by painters the harmony of colouring and the richness of effect, as exhibited either in pictures, or in the arrangements of drapery and furniture, depend upon this affection of the eye,

* Mem. Acad. pour 1743, p. 147. et seq. They had been previously described by Jurin, but only in an imperfect manner; see his essay at the end of Smith's Optics, § 260. . 6, p. 169.

† Journ. Phys. t. xxvi. p. 175. 273. et seq.; Do. p. 291. et seq. Phil. Trans. for 1786, p. 313. et seq.

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the brilliancy of the colours being much increased by the position in which they stand with respect to each other.

Besides the effect arising from the permanency of the impression after the removal of the exciting cause, there is another principle, to which this peculiar affection of the retina may be partly referred, that a nerve is unable to persevere in the same kind of action beyond a certain period, in consequence of the occurrence of what has been termed exhaustion.* The term was originally derived from the hypothesis of the animal spirits, proceeding upon the idea of there being a limited supply of these spirits in the nerve, which, by a too long continuance of the action, was suspended. The hypothesis itself being without foundation, the explanation that is derived from it must necessarily be so likewise, but in whatever manner we may explain it, the fact is one of constant occurrence, and it frequently assists us in determining whether an action is to be originally referred to the operation of the muscles or the nerves.

By combining these two principles or properties of the nervous system, we seem to obtain an easy method of explaining the various appearances which are presented by the ocular spectra. In the first case, where we have simply the effect of a greater or less degree of illumination, we may naturally ascribe the effect to the exhaustion of those parts of the retina which had been more strongly excited by the greater force of the

* Darwin classes the spectra under the two heads of direct and reverse, the first depending upon the permanence of the impression, the second upon exhaustion; Phil. Trans. for 1786, p. 313. et seq., and there appears a real foundation for this distinction. Perhaps the phenomena that are described by Dr. Brewster, designated "affections of the retina, as exhibited in its insensibility to indirect impressions, and to the impressions of attenuated light," Journ. of Science, v. iii. p. 280. et seq., may be, in some measure, explained by a reference to the effects of exhaustion and reaction.

impression made upon them. And in the same way we may explain the variations of colour that occur in the second case; for we shall find that the spectral colour is, in every instance, that which would result from a union of all the prismatic colours, except the one to which the eye had been previously exposed, and to the action of which it had consequently become more or less insensible.

It is probable that the formation of these spectra in the eye have frequently given rise to a belief in supernatural appearances. In certain diseased states of the nervous system, the retina is more than usually disposed to retain these impressions, so that, for a long time after the exciting cause has been removed, the spectrum will still remain visible.* The same causes which tend to weaken the nervous system, frequently produce a similarly debilitating influence over the mental powers, so as to render them peculiarly susceptible of being affected by superstition and credulity. The surprise which such appearances must occasion to those totally ignorant of their nature, the terror which is often associated with darkness, concurring with the weakened state of the mind and body, must be conceived, in many cases, adequate to produce the effect, without having recourse to the idea of any intentional deception on the part of the individual concerned, or of the miraculous interference of supernatural agency.†

Another circumstance which regards the operation of the nervous system, and which has been thought to

* Dr. Alderson has made use of this principle in his ingenious "Essay on Apparitions," and it has been since employed in the same way by Ferriar, and by Dr. Hibbert, in their works on the same subject. We are by this means not unfrequently enabled to explain certain supposed supernatural appearances, the evidence of which is too direct for us to doubt of their actual occurrence, without setting aside all human testimony.

†The remarks of Dr. Brewster referred to above, tend to illustrate this subject; Journ. v. iii. p. 290, 1.

favour the hypothesis of vibrations, is, that the power of a nerve in transmitting impressions is destroyed by pressure, while, by the removal of the pressure, the part regains its power, provided its structure be not injured. Now, as we have no proof of the existence. of any substance being connected with the nerve or attached to it, which can be regarded as the efficient cause of sensation, it would seem that the effect must be referred to the relation of the different parts of the nerve to each other, and this, it is conceived, may be ultimately resolved into a certain kind of motion among the particles, which motion is successively propagated from one to the other, and is counteracted by pres

sure.

It has been further urged in support of the opinion, that nervous action essentially consists in vibrations, that besides light, which is the specific and appropriate cause of vision, the sensation of sight may, under certain circumstances, be produced by other causes, which may all of them be ultimately referred to motion. A smart blow on the eye, friction and pressure upon the ball, and electricity, all produce this effect. It is difficult to conceive how a ray of light, mechanical violence, and electricity, can all have the same action upon the eye, and it may be inferred, that the only common principle on which they can operate, is the production of a certain kind of motion in the retina. and the optic nerve. Of the nature of this motion, however, either as inferred from experiment or from hypothesis, it is impossible for us to form any conception; the attempt of Hartley to reduce it to a regular system of vibrations, does not tend to throw any real light upon its nature, while I conceive that it is clearly

*Newton's Optics, Quær. 16. Op. t. iv. p. 222. A curious, and, as it would appear, an accurate account of the effect of strong pressure upon the eye ball is given us by Elliot, in his "Observations on the Senses of Vision and Hearing," p. 2, 3,

disproved by the discovery of Dr. Philip, that the action can be propagated across the interval of a divided

nerve.

*

There is a singular state of vision, which must be noticed in this place, where the eye exercises its function in a perfect manner, as far as respects the form and position of objects, and even the quantity of light that falls upon their different parts, but produces only an imperfect conception of colour. It would appear, that in this condition of the organ, there is not properly a confusion of colours, but that there is either a total incapacity of perceiving colour generally, or an insensibility to perceive certain colours, while there is a sufficiently distinct perception of others.

Numerous cases of this kind are upon record,† and we have a minute description given us by Mr. Dalton of this peculiar defect, as existing in his own eyes. He informs us, that when he looks at the prismatic spectrum, he can only distinguish three colours, which would appear to be blue, yellow, and purple, while he is incapable of perceiving either the green or the red rays. The cause of this defect is not known; we

* See v. i. p. 193, and v. ii. p. 322.

† One of the earliest is in Phil. Trans. for 1777, p. 260. et seq. by Huddart; the person of whom he gives an account, seems to have had a very clear conception of figure, and of light and shade, but probably no idea of colour of any description.

Manch. Mem. v. v. p. 28. et seq. Mr. Dalton ascribes the defect in his vision to one of the humours of his eye being "a coloured medium, so constituted as to absorb red and green rays principally" p. 42. but, I believe, that this explanation is not considered as satisfactory. He gives an account of another case, p. 37.. 41. We have two cases by Dr. Nicholls, Med. Chir. Tr. v. vii. p. 477. et seq. and v. ix. p. 359. et seq.; and one by Dr. Butter, in Edin. Phil. Journ. v. vi. p. 135. et seq.; he conceives it to be a physiological and not an optical defect, while Dr. Brewster, in his remarks on the case, supposes that it depends upon a want of sensibility in the retina, analogous to the insensibility of the ear to certain sounds. We have also a case by Mr. Harvey, in Edin. Phil. Trans. v. x. p. 253. et seq.