"as the unknown cause of a known sensation." One of the quibbles or paradoxes of the scholastic philosophy was, denying the real existence of these secondary qualities of matter. Every one is familiar with the humorous account given in the "Guardian" of the attainments of a youth from college, and his display of them when on a visit to Lady Lizard, his mother. "When the girls were sorting a set of knots he would demonstrate to them that all the ribands were of the same colour, or rather of no colour at all. My Lady Lizard herself, though she was not a little pleased with her son's improvement, was one day almost angry with him; for, having accidentally burnt her fingers as she was lighting the lamp for her teapot, in the midst of her anguish Jack laid hold of the opportunity to instruct her that there is no such thing as heat in the fire." Such speculations, which were at one time common in the schools of philosophy, had their origin entirely in an abuse of terms. The term heat, for example, has two meanings, which are quite distinct from each other. It means a sensation produced in a sentient being, and in this sense it may be said with truth that there is no heat in the fire; but it means also a quality in material substances capable of producing this sensation, and it is in this sense that we speak of heat as a property of matter.

The process by which we acquire a knowledge of external things is usually divided into two stages, namely, sensation and perception; the former implying the corporeal, the latter the mental part of it. Others apply the term perception to both; and, according to Dr. Brown, sensation is the simple impression made upon the organs of sense; perception is an association formed between this impression and an external substance which we have ascertained to be concerned in producing it. Our senses, by which this knowledge is acquired, are generally

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reckoned five, namely, sight, hearing, taste, smell, and touch. Dr. Brown proposes to add our muscular frame, and apparently with good reason; for there seems ground for believing that it is by resistance to muscular action that we acquire the notion of solidity, and that this could not be acquired by touch alone.

Our first impression of the existence and solidity of material objects, then, seems to be derived from touch combined with muscular resistance; and at the same time we acquire the knowledge of temperature, roughness or smoothness, &c. There has been some difference of opinion in regard to the manner in which we acquire the notion of extension, including figure and magnitude. It is evident that it cannot be acquired from touch alone; but it may be acquired from touch combined with muscular motion, as when we move the hand over the surface of a body. This, however, includes also the idea of time, for our notion of the extent of a surface when the hand moves over it is very much influenced by the velocity with which the motion is made. Hence time has been supposed by some to be one of our very earliest impressions, and antecedent even to the notion of extension or space. It is probable, however, that the notion of extension may also be acquired in a more simple manner from the combined operation of touch and vision. If this opinion be correct, it will follow that our first knowledge of the existence and essential properties of material things is derived from the combined operation of sight, touch, and muscular action.

With regard to all our senses, however, the truth seems to be, that the first notions conveyed by them are of a very limited and imperfect kind; and that our real knowledge is acquired only after considerable observation and experience, in the course of which the impressions of one sense are corrected and assisted by those of others, and by a process of

mind acting upon the whole. The primary objects of vision, for example, seem to be simply light or colour, and expansion. But the judgments which we are in the daily habit of forming upon vision are of a much more extensive kind, embracing also distance, magnitude, and what has been called tangible figure, such as the figure of a cube or a sphere. This last, it is evident, cannot be considered as a primary object of vision, but as entirely the result of experience derived from the sense of touch; for we never could have formed any conception of the figure of a cube or a sphere by vision alone. Distance and magnitude, also, are evidently not the primary objects of vision; for persons who have been suddenly cured of congenital blindness, by the operation for cataract, have no conception of the distance or magnitude of objects; they perceive only simple expansion of surface with colour. Our judg ment of distance and magnitude by vision, therefore, is an acquired habit, founded upon the knowledge which we have received by other means of the properties of the objects. Accordingly, it is familiar to every one, that we have no idea of the distance of an object, except we have some notion of its magnitude; nor, on the other hand, of its magnitude, except we have some knowledge of its distance. The application of this principle is also familiar in perspective drawing, in which the diminished size of known objects is made to convey the notion of distance. On the same principle, known objects seen through a telescope do not appear to be magnified, but to be brought nearer. In the same manner with regard to sounds; we have no idea of their intensity, except we have some notion of their distance, and vice versa. A given degree of sound, for example, if we believed it to have been produced in the next room, we might conclude to proceed from the fall of some trifling body; but if we supposed it to be at the distance of several miles, we should immediately

conclude that it proceeded from a tremendous explosion.

In regard to certain small distances, however, there is a power of judging by sight alone; and it appears to arise out of the degree of inclination which is given to the axis of vision in directing the two eyes to the object. Thus, in snuffing a candle, or carrying the finger to a small object within arm's length, it will be found that we are very apt to miss it, if we look with one eye only, but can touch it with unerring certainty when both eyes are directed to it. It appears to be on the same principle that we enjoy in a greater degree the deception produced by, a painting, when we look at it with one eye, especially if we also look through a tube. By the former we cut off the means of correcting the illusion by the direction of the axis of vision; and by the latter we remove the influence of all neighbouring objects. It is impossible to determine the precise distance to which we can extend this power of judging of distance by the inclination of the axis of vision, but it does not appear to be great; and in regard to all greater distances the judgment by vision is evidently an acquired habit, arising out of such a mental exercise as has now been referred to.

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 assume 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 un

common 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 deceived, 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