cannot at present be known. It must, however, be admitted that there are difficulties connected with the other hypothesis, that light consists of particles of matter sent off from the luminous body in straight lines, which are scarcely less formidable.

The apparatus provided for collecting the rays of light and bringing them into contact with the optic nerves is the eye. This beautiful instrument consists of a hollow opaque sphere, placed in a circular aperture in the skull, having a small circular hole in the front of it for the purpose of admitting the rays of light into the interior of it. This circular hole, which is the pupil of the eye, has in connection with it a series of refracting media by which the rays of light entering it are collected on the surface of the retina, where an image of the external object from which the rays of light emanate is formed. The extremity of the optic nerve spread over the surface of the retina there meets with the rays of light forming the image of the object, and sensation is produced. The hollow sphere is furnished with muscles. and every thing necessary for moving it in every direction, protecting it from injury, cleansing and moistening it, and thus fitting it for its important functions.

It has been supposed that the sense of sight intimates to the mind the direction from which the light proceeds. That the eye discerns the direction from whence light proceeds to it with much more precision than the ear discerns the direction from which the vibration of the air causing the sensation of sound proceeds, is certainly true. But we believe that this more exact discernment is not produced by the mere sense of sight, but by the muscular power of the eye in turning the pupil towards any luminous object. But the whole subject of perception by the senses will be more carefully considered as we proceed.

Three of these organs of sense have a double apparatus at the external extremity of the nerves of sense for

aiding and regulating its access to the material substance calculated to affect them,—namely, the sense of smell, which has the two nostrils; the sense of hearing, which has the two ears; and the sense of sight, which has the two eyes. The sense of taste has only one such medium or apparatus for communicating with external things, the tongue; and the sense of touch has no particular apparatus of that kind, the extremities of the tactual nerves being, as we have said, spread over the whole surface external and internal of the body. The double apparatus possessed by three of the senses does not cause double sensation, but only strengthens the one sensation. The two nostrils do not cause a double, but a stronger smell; nor do the two ears cause a double sound, but strengthen the single sound. So, in a healthy and natural state of the eyes, the sensations produced by them are not double but single. But some diseases of the eye, or pressure on one of the eye-balls forcing it out of its natural position, cause double sensation. If the two eyes be directed to a very near object, more distant objects may be seen double; or if the eyes be directed to a distant object, and a near object, such as the finger, be interposed between them and the distant object, a double image of the near object may be seen. Thus, if both a near and a distant object be before the eyes at the same time, the near object may be seen double by directing the eyes to the distant object, or the distant object may be seen double by directing the eyes to the near object. Or, if the same distant object be seen by the one eye through a magnifying prospect-glass, and by the other naked, two images of the object will be seen; the one larger than the other, which, by a little management, may be brought near to one another, and compared, so as to enable the observer to measure the magnifying power of the prospect-glass. Or, the same object may be viewed by two prospect-glasses at the same time applied to the two eyes, and double images of the object

produced, which may be compared, and the comparative magnifying powers of the two instruments ascertained. From such phenomena as these it has been conjectured that there are corresponding points on the retina of each eye on which, if a pencil of light fall, it produces but one sensation, but that if a ray of light from any object be made to fall on points which do not correspond, double sensation is produced. No artificial effect on the ears or the nostrils has ever been found to cause a double sensation of sounds or smells; nor are we aware that any disease of these organs has ever caused such a phenomenon.

CHAPTER II.

GENERAL OBSERVATIONS ON THE SENSES.

1. EACH of these five organs of sense yield many different sensations; the taste distinguishes many different savours; the smell many different odours; .the hearing many different sounds; the touch many different degrees and modifications of impact or pressure, and many different degrees of heat and cold; and the sight many different degrees of light and shade, and many different colours.

2. The various sensations caused by the different qualities in external objects, form the only means of communication which the mind possesses with the material world. They have been compared to the windows through which the mind looks out upon external objects; but they are rather of the nature of apertures through which the material world finds access to the mind. The activity in establishing the communication is not in the mind, but in the external objects. It is the effluvia emanating from odoriferous bodies entering the nostrils; the undulations of the air from a sonorous body entering the ears; the rays of light from a luminous

body entering the pupils of the eyes, that awaken the mind to sensation, and not the mind already awake and active, looking out for external objects. In two of the

senses, the substances fitted to create sensation must be brought into actual contact with the organ before sensation is produced. When the mind is affected by objects of sense, it becomes active within itself, as we are to explain in the sequel; but originally, the sensation comes to the mind from without, and not the mind to the object of sensation.

3. These sensations form the elements and origin of all thought, all pleasure or pain, and all intellectual operations. The mind once affected by them, works them up into a prodigious variety of most interesting and important operations, by comparing, discriminating, remembering, arranging, drawing inferences,-all the variety and complexity of mental operations. For apart from sensations, and the operations of the mind upon them, the mind would remain as dormant as when the body is in the womb.

For

4. Most, or all of these classes of sensations, seem. to be capable of being excited by other means than those which form the usual causes of them. example, certain diseases produce sensations of sound or noise that cannot be distinguished from the sensations produced by sounding bodies. Pressure, or impact on the eye, produces sensations similar to those produced by luminous objects. Whether the illusions produced by such diseases as delirium tremens be caused by effects on the visual nerves, or by some other internal cause unconnected with the nerves, must be left to the researches of pathologists; but that these phenomena are produced by the effects of disease on the visual nerves seems most probable, inasmuch as the illusions consist of images of visible objects. Some diseases in various parts of the body, external and internal, produce sensations that so closely resemble the sensations

caused by the pressure of external substances, or the application of sharp edges or sharp-pointed instruments, or the application of fire or of heated substances, that patients labouring under these affections always describe them by reference to the pain caused by such external applications; they say they feel as if there were a weight resting on the part, or as if it were pierced with a sharp instrument, or as if it were burnt. Whether unpleasant smells and tastes may not arise from diseases directly affecting the nerves of taste or smell, or whether these sensations may not proceed from odours or tastes actually coming up from the interior of the body to the palate and the olfactory nerves, is another question that belongs to pathology. There, however, can be no doubt that some sensations can be produced otherwise than by the ordinary process originating in an external body communicating with the mind through the organ

of sensation.

5. When different organs of sense are affected at the same time—as the ear with sounds, the eye with light, .the nostrils with odour, the sense of touch by contact with any substance; or when the same organ of sense is affected at the same moment by different objects-as the ear by different sounds, the eye by different colours, the nostril by different odours-the result is not one sensation modified by these different causes of sensation, nor a multitude of sensations affecting the mind simultaneously but the effect is, that one sensation occupies the mind to the exclusion of the others, the mind being rendered for the time insensible to the other effects produced on the organs of sense. The causes that may give to one sensation its preponderating power to possess the mind exclusively, are various. Intensity is one of the most prevalent. A loud noise, a dazzling light, an intense odour, or savour, will occupy the whole mind for a time, precluding other sensations, although other effects are being at the moment produced on the