they suggest the idea of roughness. The different positions of resisting points and surfaces with respect to each other, suggest the idea of locality, together with those of diversity and of number; and the continued and repeated experience of resistance at the same point, on the same surfaces, and in the same planes, suggests the idea of identity. Ideas of extension are inferences from those of locality. These and other similar ideas are inferred from sensations of touch. Many of them may also be inferred from other sensations. Ideas of these kinds are intimately associated with the sensations from which they are derived, and are usually entertained mainly with reference to the objects of perception.

Sight.

§ 33. The exclusive organ of sight is the eye. The lenses of this organ serve as mediums of refraction to concentrate the rays of light, as they proceed in diverging lines from different points of visible bodies. Each point of a visible body, sends out diverging rays or streams of light, which the lenses of the eye render converging, and bring to so many corresponding points of the retina. The retina of the eye is an expansion of the optic nerve, which covers the posterior part of the globe of the eye, and serves as a screen to receive the light, the rays of which meet in points upon its surface. Every point in a visible body is repre sented on the retina, by the whole number of rays from that point, which enter the pupil of the eye. The pupil of the eye is the dark circular space directly in the front, and is surrounded by a colored net-work, called the iris, which expands and contracts on its inner surface, so as to enlarge the pupil when the light is weak, and contract it when the light is strong. This office of the iris accommodates the eye, as far as possible, with that amount of light which is best suited to the purposes of vision. When the light is too strong, the iris expands on its interior surface, and diminishes the size of the opening within it, so as to exclude the excess; when the light is too weak for perfect vision, it contracts, and thus enlarges the aperture for the admission of light, and proportionably increases the amount received. The pupil of the eye, therefore, through which light is admitted for the purpose of vision, is subject to constant variations in size, depending upon the amount

of light received. This arrangement is one of the most beautiful contrivances of Divine wisdom for the adaptation of the eye to its manifold and continual uses.

§ 34. Vision is generally explained as the effect of the formation of pictures on the retina of the eye, by the pencils of light which proceed in diverging lines from all points of visible bodies, and are there converged to corresponding points again. This is a mistake. The picture does not exist as the condition of sensation, but as its effect. The conditions of the sensations peculiar to the eye, are not pictures, but concentrated streams of light, issuing from all points in the surfaces of visible bodies, and meeting in corresponding points on the retina of the eye. The effects of these pencils of light, thus impinging on the retina, are sensations of color, embracing feelings of all the different varieties of light and shade. These sensations are different in every point of the retina on which light from visible objects is received. The mind, by an act of judgment, connects them together, refers them to their external exciting causes, and forms corresponding conceptions of the objects from which they proceed. Visible objects, and pictures of visible objects, exist absolutely as reflectors of light. But vision is an effect, not of pictures, but of light acting on its appropriate organ, just as sound is an effect of vibrations which act on the organ of hearing; and touch, of resistances on the organ of touch. Pictures are things to be perceived by sight, not means of perception. Light, however, impinging on the retina of the eye is, by its action on that organ, a means of the sensations of color, embracing all the infinite diversities of light and shade. From these sensations the mind forms its ideas of colors, and of different lights and shades, and also of external colored objects.

Sensations experienced by the eye are the foundation of the ideas of color. The reference of colors from the eye, where they are felt to external objects at greater or less distances from us, is one of the most remarkable instances of the exercise of reason and association, within the range of human experience. Color is felt only in the eye, but always referred to external objects, from which the light producing it proceeds. This reference is a combined act of judgment and association. Impressions produced by light become associated with the sizes, forms, positions and texture of bodies, so that the moment they occur, we immedi

ately form corresponding judgments, embracing their various elements.

§ 35. Vision consists of several consecutive and contemporaneous judgments, based on sensations of color, and having relation to the existence, forms, magnitudes, distances and nature of the bodies seen.

Sensations of color are referred to their external causes, and those causes identified with the causes of other sensations, to the greatest extent possible.

All that the eye gives us is sensations produced by light. It gives neither form, size, position, nor any other property of visible bodies. From these sensations all ideas of visible objects are inferred, many of them instantaneously, others more slowly and with more deliberation; and all, in most cases, with a clearness and definiteness of conception, that fill us with wonder and admiration.

Every thing pertaining to sight is interesting; the structure of the eye; the endless diversity of impressions produced by light; the extremely refined nature of that substance by which it is adapted to purposes of vision, together with the speed and extent of its travels, and the accuracy of its intimations. What an exhibition is it of the wisdom and power of the Creator! What a monument is it of the Creator's goodness! He that said "Let there be light, and there was light;" He that called this substance into being, and adapted it to its purposes; and formed the eye to be the mind's organ of appreciating it, is to be recognized as marvelous in his works.

The structure of the eye is scarcely less wonderful than the properties of light which it reveals. Its achromatic combination of lenses, so as to refract light without dispersing the rays of different colors; the delicate structure of all its parts; their location in proper positions; the variable size of the pupil; and the beautiful iris, with its rainbow brilliancy, are all master-pieces of Divine contrivance and workmanship.

Hearing.

The external ear

§ 36. The organ of hearing is the ear. consists of a fibrous, elastic cartilage, which is attached to the side of the head by a cellular tissue and by muscles. The muscles of the human ear are but imperfectly developed. This organ is characterized by five eminences and

three cavities. From the external ear there is an aperture extending to the internal, which, in adults, is about three quarters of an inch long, and at its termination is the tympanum or drum of the ear, which separates the external from the internal ear. Beyond the tympanum is the internal ear or labyrinth, composed of several cavities communicating with each other, covered with a thin membrane, filled with a thin liquid, and pervaded with the acoustic nerve. The internal and middle ear are traversed by several other nervous threads, besides those of the acoustic nerves. The acoustic nerves are the seventh pair, not much larger than sewing threads, before entering the ear, but distributed after entering it, under a variety of forms in all the different tubes, sacs, canals, and pits of this organ.

§37. The sensations peculiar to the ear are those of sound. They are produced by vibratory motions of sounding bodies, often imperceptible to the eye and touch, communicated to the air, and from the air to the tympanum and other parts of the internal ear. The effects of these vibratory motions on the ear, are sounds. Sounds, therefore, are sensations corresponding to certain vibratory motions of sounding bodies. Like other sensations they belong exclusively to the mind, but are indexes of things without, and answer many important purposes, both as means of knowledge and delight.

§38. The principal qualities of sound are tone and intensity. Tone depends on the number of vibrations made by sounding bodies in a given time. A certain number of vibrations in a given time, produce sounds of a certain tone; a greater number in the same time, produce sounds of greater acuteness; and a less number, those which are less acute or more grave. The most grave sounds which the ear is capable of appreciating are produced by 32 vibrations a second; the most acute are the effect of 12,000. The interval between the lowest, and the most acute sounds appreciable by the human ear, is filled up with an immense variety of sounds of an intermediate character. The variations of sounds in respect to tone depend on the rapidity of the vibrations which produce them. Those produced by vibrations which are equally rapid, agree in tone; and those which are produced by vibrations of unequal rapidity, disagree in tone.

§ 39. Two principal uses of sounds are those which relate to music and speech. Musical sounds are divided into octaves or series of sounds consisting of eight sounds each. The larger intervals between one of these sounds and the next are called tones, and the smaller semitones. Every octave contains five tones and two semitones. The vibrations which produce the first sound of an octave are precisely half as rapid as those which produce the last; and when these two sounds are produced together, every other vibration of the body producing the most acute sound will coincide with every vibration of that producing the most grave, forming an accord. The most perfect accord, however, is produced by sounds of the same pitch, in which case the vibrations producing both, agree perfectly and concur in producing one stronger set of vibrations. This accord is called a unison. The other principal accords within the range of a single octave, are the first sound and the third, and the first and fifth. In all accords the vibrations of the

sounding bodies are partly together. In some cases every second vibration of one agrees with every third of the other; and in others, every third vibration of one agrees with every fifth of the other; and so on.

Sounds which do not accord when produced together, are called discords. Accords produce agreeable sensations, discords disagreeable ones. Sounds produced by the vibrations of complex bodies, are often of a complex character in respect to tone; one part vibrating more rapidly, and others less. Sounds of this description are not adapted to music, and are less agreeable than simple ones, for the purposes of speech.

§40. The constitution of bodies so that vibrations adapted to produce sounds should be continually and easily produced, the adaptation of the ear to appreciate these vibrations, and the capacity of the mind to experience from them the appropriate sensations, are among the wonders of the human and animal constitutions. External bodies act on the ear by vibrations, and the effects are corresponding sensations of sound. The reference of sounds to external objects which produce them, is an act of judgment similar to the reference of other sensations to external objects.

$41. Music consists mainly in modifications of sounds in respect to tone and time. Time is scarcely less important to music than tone. Musical sounds are required to be sim