There are some other circumstances, also, the result of experience, by which we are greatly influenced in all such cases, particularly the degree of illumination of the objects, and the degree of distinctness of their outline and minute parts. Thus, in a picture, distant objects are represented as faintly illuminated, and with indistinctness of outline and minute parts; and vice versa. On this principle, objects seen through a fog, or in obscure light, are apt to appear much larger than they really are; because, in the mental process which takes place in regard to them, we first as sume them to be distant, from their imperfect outline and faint illumination, and then, judging from this assumed distance, we conclude them to be of great size. On the other hand, objects seen in an unusually clear state of the atmosphere appear nearer than they really are, from the greater distinctness of their outline. In our judgment of distance by sight, we are also greatly influenced by the eye resting on intermediate objects; and hence the difficulty of judging of distances at sea. A striking illustration of the same principle is furnished by captain Parry, in regard to objects seen across a uniform surface of snow. "We had frequent occasion, in our walks on shore, to remark the deception which takes place in estimating the distance and magnitude of objects, when viewed over an unvaried surface of snow. It was not uncommon for us to direct our steps towards what we took to be a large mass of stone, at the distance of half a mile from us, but which we were able to take up in our hands after one minute's walk. This was more particularly the case when ascending the brow of a hill." Captain Parry adds, that this deception did not become less on account of the frequency with which its effects were experienced; and a late writer has used this as an objection to the doctrine lately referred to, respecting the influence of experience on our judgment of distance by vision. But this is evidently founded on a misconception of the effect of experience in such cases. Captain Parry could mean only, that he did not acquire the power of judging of the distance or magnitude of unknown objects. Had he been approaching an object by which he had once been de

Effects of distance-what? Illustration from Parry's Journal. The deception not diminished by experience. Reason.

ceived, knowing it to be the same, he would not have been deceived a second time; but, judging from its known magnitude, would have inferred its distance. Thus the result of experience is to enable us to judge of the distance of an object of known magnitude, or of the magnitude of an object at a known distance; but, in regard to objects of which both the distance and magnitude are unknown, it teaches us only not to trust the indications of vision.

In our judgment of vision by the magnitude of objects, again, we are much influenced by comparison with other objects, the magnitude of which is supposed to be known. I remember once having occasion to pass along Ludgate Hill, when the great door of St. Paul's was open, and several persons were standing in it. They appeared to be very little children; but, on coming up to them, were found to be full-grown persons. In the mental process which here took place, the door had been assumed as a known magnitude, and the other objects judged of by it. Had I attended to the door being much larger than any door that one is in the habit of seeing, the mind would have made allowance for the apparent size of the persons; and, on the other hand, had these been known to be full-grown persons, a judgment would have been formed of the size of the door. On the same principle, travellers visiting the pyramids of Egypt have repeatedly remarked, how greatly the notion of their magnitude is increased by a number of large animals, as camels, being assembled at their base.

There is something exceedingly remarkable in the manner in which loss or diminution of one sense is followed by increase of the intensity of others, or rather, perhaps, by an increased attention to the indications of other senses. Blind persons acquire a wonderful delicacy of touch; in some cases, it is said, to the extent of distinguishing colors. Mr. Saunderson, the blind mathematician, could distinguish by his hand, in a series of Roman medals, the true from the counterfeit, with a more unerring discrimination than the eye of a professed virtuoso; and, when he was present at

What is really gained by experience. Influence of comparison in judgment by vision. Illustration. Explanation. Illustration from the pyramids. Effect of the loss or dimi nution of a sense. Examples. Saunderson.

the astronomical observations in the garden of his college, he was accustomed to perceive every cloud which passed over the sun. This remarkable power, which has sometimes been referred to an increased intensity of particular senses, in many cases evidently resolves itself into an increased habit of attention to the indications of all those senses which the individual retains. Two instances have been related to me of blind men who were much esteemed as judges of horses. One of these, in giving his opinion of a horse, declared him to be blind, though this had escaped the observation of several persons who had the use of their eyes, and who were with some difficulty convinced of it. Being asked to give an account of the principle on which he had decided, he said it was by the sound of the horse's step in walking, which implied a peculiar and unusual caution in his manner of putting down his feet. The other individual, in similar circumstances, pronounced a horse to be blind of one eye, though this had also escaped the observation of those concerned. When he was asked to explain the facts on which he founded his judgment, he said he felt the one eye to be colder than the other. It is related of the late Dr. Moyse, the well-known blind philosopher, that he could distinguish a black dress on his friends by its smell: and there seems to be good evidence that blind persons have acquired the power of distinguishing colors by the touch. In a case of this kind, mentioned by Mr. Boyle, the individual stated that black imparted to his sense of touch the greatest degree of asperity, and blue the least. Dr. Rush relates of two blind young men, brothers, of the city of Philadelphia, that they knew when they approached a post in walking across a street, by a peculiar sound which the ground under their feet emitted in the neighborhood of the post; and that they could tell the names of a number of tame pigeons, with which they amused themselves in a little garden, by only hearing them fly over their heads. I have known several instances of persons affected with that extreme degree of deafness which occurs in the deaf and dumb, who had a peculiar susceptibility to particular kinds

Two blind men. Dr. Moyse. Instances adduced by Dr. Rush. Certain sounds perceived by the deaf.

of sounds, depending apparently upon an impression communicated to their organs of touch or simple sensation. They could tell, for instance, the approach of a carriage in the street, without seeing it, before it was taken notice of by ⚫persons who had the use of all their senses. An analogous fact is observed in the habit acquired by the deaf and dumb, of understanding what is said to them by watching the motion of the lips of the speaker. Examples still more wonderful are on record, but certainly require confirmation. A story, for instance, has lately been mentioned in some of the medical journals, of a gentleman in France who lost every sense, except the feeling of one side of his face; yet it is said that his family acquired a method of holding communication with him, by tracing characters upon the part which retained its sensation.

Much ingenuity has been bestowed upon attempts to explain how, with two eyes, we see only one object; and why that object is seen erect, when we know that the image on the retina is inverted. All that need be said upon the subject, and all that can properly be said, appears to be, that such is the constitution of our nervous system. It is on the same principle, that by the sense of touch, in which may be concerned a thousand or ten thousand distinct points of contact, we receive the impression of only one body; or, what perhaps may appear a more strictly analogous case, we receive the impression of but one body, though we grasp the substance with two hands, or with ten distinct fingers. For the healthy perception in both these cases, however, a certain arrangement is required, which we may call the natural harmony of the nervous system; and when this harmony is disturbed, the result is remarkably altered. Thus, squinting produces the vision of a double image,* because the images fall upon what we may call unharmonizing points of the retina; and the same principle may be illustrated in a very curious manner by a simple experiment with the

• This effect may easily be produced by pressing one of the eyes a little out of its natural position by means of the finger at the corner of it, while looking at a single object. It will be made to appear double.

Extraordinary case of a gentleman in France. object appears single and direct. Analogous case.

Difficulty of explaining why the
Effect of squinting, what?

sense of touch. If a small round body, such as a pea, be laid upon the palm of the one hand, and rolled about between the first and second fingers of the other, in their natural position, one pea only is felt; but, if the fingers are crossed, so that the pea is rolled between the opposite surfaces of the two fingers, a most distinct impression of two peas is conveyed.

Of the whole of the remarkable process of sensation and perception, we know nothing but the facts, that certain impressions made upon the organs of sense are followed by certain perceptions in the mind; and that this takes place, in some way, through the medium of the brain and nervous system. We are in the habit of saying, that the impressions are conveyed to the brain; but, even in this, we probably advance a step beyond what is warranted. We know that the nerves derive their influence from their connection with the brain, or as forming along with it one great medium of sensation; but we do not know whether impressions made upon the nervous fabric connected with the organs of sense are conveyed to the brain; or whether the mind perceives them directly, as they are made upon the organs of sense. The whole subject is one of those mysteries which are placed above our reach, and in which we cannot advance a single step beyond the knowledge of the facts. Any attempt to speculate upon it is therefore to be considered as contrary to the first principles of philosophical inquiry. We must simply receive the facts as of that class which we cannot account for in the smallest degree; and the evidence which we derive from our senses, of the existence and properties of the things of the material world, is to be recog nised as one of those fundamental laws of belief which admit of no other proof than that which is found in the universal conviction of mankind.

Before concluding the subject of perception, it remains to be noticed that a certain voluntary effort is required for the full exercise of it; or, at least, for that degree of perception which leaves an impression capable of being retained. It is familiar to every one, that when the mind is closely occupied, numerous objects may pass before our eyes, and cir

Experiment with the touch. Extent of our knowledge of sensation. The brain. Difficulty of the subject. Voluntary effort necessary. Evidence of it.