intensity, at a given distance, is unknown, as, for example, thunder, we suppose it nearer according as it is louder.'

We are apt to mix inferential processes with our judgment of distance. If we are led to imagine that a sound is farther off than it really is, we seem to hear it stronger than it is. Awaking suddenly in the night, we hear a faint noise, and suppose it much louder, our notion of its real distance being for a few moments vague and confused. It being an effect of distance that sounds fade away into a feeble hum, when we encounter a sound whose natural quality is feeble, like the humming of the bee, we are ready to imagine it more distant than the reality.

14. Direction. This is a purely intellectual sensation, in other words, is of importance as leading us to perceive the situation of the objects of the outer world whence the sound takes its rise.

The following extract from Longet indicates the kind of experience that gives us the feeling of direction :

'With regard to the direction of the sonorous waves, we can at present only say, that the knowledge of it is owing to a process of reasoning applied to the sensation. Thus, we hear distinctly a sound emanating from a given point, whatever be the position of the head; but the ear being able to judge of slight differences in the intensity of sounds, we remark that, in certain positions of the head, the sound seems stronger. We are hence led to place our head in one fixed position as regards the sounding body. But our sight tells what is this direction of most perfect hearing; and we then apply the observation made on bodies that we can see to those that are not seen.'

The combined action of the two ears also favours the perception of direction of sound very materially. A person that has become deaf on one ear, is usually unable to say whether a sound is before or behind. The change of effect produced by a slight rotation of the head, is such as to indicate direction to the mind. For while the sound becomes more perceptible on one ear,-the ear turned to face the

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object more directly, the sound in the other ear is to the same degree obscured. When the head is so placed, after various trials, that the greatest force of sensation is felt on the right ear, and the least on the left, we then infer that the sounding body is away to the right; when the two effects are equal, and when any movement of the head makes them unequal, we judge the sound to be either right in front or behind; and we can further discriminate so as to determine between these two suppositions.*

The sense of direction is by no means very delicate, even after being educated to the full. We can readily judge whether a voice be before or behind, right or left, up or down; but if we were to stand opposite to a row of persons, at a distance, say, of ten feet, we should not be able, I apprehend, to say which one emitted a sound. This confusion is well known to schoolmasters. So it is next to impossible to find out a skylark in the air from the sound of its song.

15. The duration of the feeling of an individual beat can be appreciated by noting at what intervals a succession of beats seems an uninterrupted stream of sound. This makes, in fact, the inferior limit of the audibility of sounds. From the experiments of Helmholtz, it would appear that a series of beats begins to be felt as continuous when they number

According to Ed. Weber, in determining the direction of sounds, we employ the external ear for those coming from above, below, behind, before; the tympanum for those coming from left to right. He made the following experiments:-The head was inserted in water, the air-passage being filled with air, so that the tympanum was free to vibrate. In that case, the ear recognized the sounds as external to itself, but could distinguish them only as right or left in direction. When, farther, the ear itself was filled with water, and the free action of the tympanum arrested, the sense of externality altogether was lost. The feelings were regarded as subjective. It was observed by E. H. Weber that the uniting of the double sensation from the two ears (analogous to binocular vision) has its limits. If two watches with different rates of ticking are held before one ear, the ear distinguishes the periods when the strokes of the two fall together, and forms to itself a rhythm out of the two series of strokes. If the watches are applied, one to each ear, the sense of rhythm is lost. The mind can no longer make the combination effected when the two watches are applied separately to the two ears.

sixteen in a second; so that the impression of each must continue not less than the sixteenth part of a second.

16. The subjective sensations of the ear are such as buzzing, ticking, and humming sounds. They arise from disease of the brain, or the auditory nerve, obstructions in the tympanum and Eustachian tube, &c.

SENSE OF SIGHT.

1. The objects of sight include nearly all material bodies. Their visibility depends on their being acted on by Light, hitherto the most inscrutable of natural agents. Certain bodies, as the Sun, the Stars, flame, solids at a high temperature, give origin to rays of light, and are called self-luminous. Other bodies, as the Moon, the Planets, and the greater number of terrestrial surfaces, are visible only by reflecting the rays they receive from the self-luminous class.

The reflexion of light is of two sorts: mirror reflexion, which merely reveals the body that the light comes from; and reflexion of visibility, which pictures the reflecting surface. In this last mode of reflexion, the light is broken up and emitted in all directions exactly as from a self-luminous original. Visible surfaces receiving light from the sun have thus the power of absorbing and re-issuing it, while a mirror simply gives a new direction to the rays. When we look at a picture in a bad light, we find that the rays of reflexion overpower the rays arising from the coloured surface of the picture; consequently the picture is imperfectly seen.

As regards vision, bodies are either opaque or transparent. There is a scale of degrees from the most perfect opacity, as in a piece of clay, to the most perfect transparency, as in air. According as bodies become transparent, they cease to be visible.

The transparency of Air is not absolutely perfect; that is to say, light in passing through the atmosphere is to a certain small extent arrested, and a portion reflected, so as to make the mass faintly visible to the eye. When we look up into

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the sky through a cloudless atmosphere, all the illumination received beyond the sun's disc is light reflected by the atmosphere itself. Liquids are still less transparent; although they transmit light so as to show objects beyond them, they also reflect a sufficient portion to be themselves visible. Light falling upon the surface of water is dealt with in three different ways. One portion passes through, a second is reflected as from a mirror, a third very small portion is absorbed and radiated anew, so as to make the surface visible as a surface. The same threefold action obtains in transparent solids, as glass, crystal, &c. It is to be remarked of solid bodies that they are almost all transparent to a certain small depth, as shown by holding up their plates or lamina to the light. Gold leaf, for example, permits the passage of light; and any other metal, if similarly attenuated, would show the same effect. There is, however, in this case, an important difference to be noted, inasmuch as objects are not distinctly seen, although light is transmitted; hence the name 'translucent' is applied to the case to distinguish it from proper transparency. There may be something more than a difference of degree between the two actions.

Opaque bodies may diffuse much light or little some substances, such as chalk and sea foam, emit a large body of light; charcoal is remarkable for absorbing without re-emission of the sun's rays. This is the ordinary, perhaps not the full, explanation of white and black, the one implying a surface that emits a large portion of the rays of visibility, the other few or none.

Besides the difference of action making white and black, and the intermediate shades of grey, there is a difference in the texture of surfaces, giving birth to what we recognize as Colour. Upon what peculiarity of surface the difference between, for example, red and blue, depends, we cannot at present explain. But this fact of colour is one among the many distinctions presented by the various materials of the globe. Along with colour, a substance may have more or less of the property that decides between white and black, namely,

copiousness of radiation. This makes richness of colour, as in the difference between new and faded colours, between turkey red and dull brick clay of a similar hue.

Some bodies are farther said to possess Lustre.

Mineral bodies present all varieties of light, colour, and lustre, but the prevailing tint of rocks and soils is some shade of grey. The reddish tint of clays and sandstones is chiefly due to the prevalence of oxide of iron. Vegetation yields the greenness of the leaf, and the variegated tints of the flower. Animal bodies present new and distinct varieties.

2. We come next to consider the organ of sight, the Eye. 'Besides the structures which compose the globe of the eye, and constitute it an optical instrument, there are certain external accessory parts, which protect that organ, and are intimately connected with the proper performance of its functions. These are known as the appendages of the eye (they have been named likewise 'tutamina oculi'); and they include the eyebrows, the eyelids, the organ for secreting the sebaceous (or oily) matter, and the tears, together with the canals by which the latter fluid is conveyed to the nose.'

"The eyebrows are arched ridges, surmounting on each side the upper border of the orbit, and forming a boundary between the forehead and the upper eyelid. They consist of thick integument, studded with stiff, obliquely set hairs, under which lies some fat, with part of the two muscles named respectively the orbicular muscle of the eyelids and the corrugator of the eyebrows.' By this last-named muscle the eyebrows are drawn together, and at the same time downwards, so as to give the frowning appearance of the eye; the opposite action of lifting and separating the eyebrows is performed by a muscle lying beneath the skin of the head termed the occipito-frontalis. In regulating the admission of light to the eye, and in the expres sion of the passions, these two muscles are called into play; the one is stimulated in various forms of pain and displeasure, the other in an opposite class of feelings.

'The eyelids are two thin moveable folds placed in front of each eye, and calculated to conceal it, or leave it exposed, as occasion may require. The upper lid is larger and more moveable than the lower, and has a muscle (levator palpebræ superi