associations is interrupted, we have double vision produced. But to this it may be answered, that we have here not a mental, but a physical defect, for if we examine the state of the eye in these instances we shall observe, that the eyes do not move in a parallel direction, and that consequently the impressions are not made upon corresponding points in the retina. Upon the same principle double vision is always produced, when, from accident or disease, the eyes are prevented from moving in concert, and however long the irregular motion is continued, it is found that the defect of vision remains, and that we never acquire the power of conceiving the impressions to be single, until the physical defect of the eye be remedied.*

In considering this question we are naturally led to enquire into the cause of the tendency which we observe in the eyes to move in the same direction; is this a natural propensity, or is it acquired by habit? Smith, in conformity with his general principles, argues in favour of the latter opinion; he remarks, that when both eyes are directed to the same object, we see it more distinctly than when viewed by one, and finding this to be the case, we insensibly acquire the custom of moving the eyes together. The contrary

* Cheselden, indeed, mentions an instance, where a person, in consequence of an injury, had the eyes distorted, and consequently experienced double vision, who afterwards gradually acquired the power of seeing objects single, although the distortion was not removed; Anat. p. 295, 6. But it may be questioned, whether in this case the sight of the distorted eye was not so far impaired, as that the patient ceased to attend to the impression. I must remark, however, that Camper admits the correctness of Cheselden's observation, and advocates the hypothesis of Smith; De Visu, in Haller, Disp. Anat. t. iv. p. 243.

† Optics, § 137. Porterfield also conceives that it depends upon habit, and in proof of his opinion, remarks, that the eye lids and other neighbouring parts have the same tendency to move in corresponding directions, a circumstance which, I apprehend, is rather unfavourable to his opinion; Edin. Med. Ess. v. iii. p. 255, also "On the Eye,” v. i. p. 118, and v. ii. p. 326.

doctrine is adopted by Reid,* and I conceive that it is sanctioned by experience; for it would appear that where the organ is sound, and there is no mal-conformation of the, neighbouring parts, the eyes will be found to have a natural tendency to move in the same direction, and that this parallelism of motion is observed in the eyes of very young children, and even of blind persons, or of those who have the sight of only one eye, in which cases we cannot suppose that it has been acquired by any operation of habit or association.†

Now if the muscles of the eye are so constituted and so connected with the nervous system, that in their natural state they have a tendency to place the eyes in such a position with respect to each other, as that the impressions of an object are formed upon corresponding parts of the two retina, it would seem to follow as a probable inference, that some further purpose was to be obtained, and that there must be a natural sympathy or connexion between the corresponding parts of the retina, which, without any mental effort, produces only a single perception. Notwithstanding, therefore, the anatomical arguments that have been adduced against Reid's doctrine of corresponding points, I am disposed to regard it as not without foundation, and that even if this cannot be maintained, that there are stronger and more direct objections against the hypothesis, which accounts for single vision upon the principle of habit and association.

It may be proper to notice in this place that peculiar state of the eyes which produces squinting, as it has been supposed to throw some light upon the theory of single vision. In the individuals who are the

* On the Mind, ch. 6. sect. 10.

† Dr. Wollaston supposes that the parallel motion of the eyes is connected with the partial union of the optic nerves; Phil. Trans. for 1824, p. 229.

subjects of this defect, the eyes do not move in the same direction, and it was supposed by many of the older physiologists, that it depended upon a want of correspondence between the retina, and that in order to produce the same effect upon each of them, it was necessary that the impressions should be made upon different parts of the surfaces. There are two points to be ascertained before we can form a correct judgment concerning the cause of squinting; do the persons who squint use both their eyes at the same time? and if so, do they see objects double? To both these questions it seems that we must answer in the negative, as we find, that when they look attentively at an object, they never use more than one eye. The immediate cause of the other eye not being directed to the object, or rather being drawn away from it, appears to depend upon its vision being imperfect, so that if it were directed to the object together with the sound eye, it would produce a confused impression, and it is to prevent this defect that the habit of turning the eye. aside is unconsciously acquired. This view of the subject was proposed by Buffon, and our subsequent observations seem to justify his opinion. Hence we perceive

* Delahire supposed that squinting depends upon the most sensible parts of the two retinæ not being similarly situated with respect to their centres; see his treatise, "Accidens de la Vue," § 10. in Mem. Acad. t. ix. p. 530. et seq.

A case which is related by Darwin, in Phil. Trans. for 1778, p. 86. et seq. seems however to prove that there are occasional exceptions to this general principle.

Mem. Acad. pour 1743, p. 231. et seq. Jurin had previously refuted Delahire's hypothesis referred to above; Essay attached to Smith's Optics, § 178..194; he ascribes it to a habit acquired early in life of directing only one eye to the object. Porterfield, who considers in detail the phenomena and causes of squinting, enumerates six different circumstances, by which he conceives it to be produced; one of these is that assigned by Buffon; Edin. Med. Essays, v. iii. p. 237. et seq. Dutours proposed a modification of this hypothesis; he supposes that one of the retinæ is, in these cases, more sensible to light than the other, and is conse

that the idea which was formerly entertained respecting the cause of squinting, as depending upon a want of correspondence between the different parts of the two retina, is without foundation, and that consequently it throws no light upon the nature of single vision.

quently turned away from the object; Mem. Present. à l'Acad. t. v. p. 470. et seq. Reid, On the Mind, sect. 16; Priestley, On Vision, per. 6. sect. 12. ch. 3; Sir Ev. Home, Phil. Trans. for 1797, p. 12.. 18; and Mr. Bell, Anat. v. iv. p. 456. et seq. adopt the opinion of Buffon.

CHAPTER XIV.

Of Hearing.

AFTER the sense of sight, that of hearing will next claim our attention, both in consequence of its real importance in the various concerns of life, and of the elaborate structure of the organ by which it is exercised. We have also a tolerably correct knowledge of the nature of its specific cause, of the mode in which the ear receives the impressions of sound, and of the manner in which they act upon it. We have, however, a much less perfect idea of the use of the different parts of the ear than of the eye, we have less command over the cause of sound, when we attempt to make experiments upon it, and we are also less able to obtain a perfect knowledge of the acquired perceptions of hearing. In this chapter I shall first give some account of the structure and functions of the ear, and shall afterwards make some remarks upon the acquired perceptions of hearing.

1. Account of the Structure and Functions of the Ear.

Sound* is excited by the vibration or oscillation of the particles of certain bodies, which, from this circumstance, are termed sonorous. They are of different kinds, and are found in all the three mechanical states in which bodies exist, of solid, fluid, and aeriform.

* For an account of the production of sound, its transmission from one body to another, and the various modifications which it experiences, it will be sufficient to refer to the learned and elaborate work of Dr. Young; Lect. v. i. No. 31..4. An ample list of references is contained in v. ii. p. 264. et seq. The authors who have treated on the "Ear and Hearing," are enumerated in p. 271, 2.