pupil contracts under the influence of the light, and this cannot occur without the co-operation of the centre of the optic nerve in the brain. The sensation is, however, only an internal one, like the feeling of satiety or of hunger; it is, of course, not yet recognised as proceeding from external objects. We observe, indeed, that a child gradually begins by moving eye and head to fix the object, namely, to bring the eye into such a position that the image of the object may fall upon the point of most distinct vision in the centre of the retina. Yet in this stage of development the transference of what is seen to the outer world has

not yet taken place. The fixing of the eye upon objects does not take place suddenly, but is gradually developed from a spasmodic attempt to move the eye, which, perhaps, is caused by the irritative effect of the light; and if during this motion the point of distinct vision chances to fall upon an object which attracts attention by its brightness, colour, or motion, then by a repetition of this process, the child will gradually learn by experiment to repeat the required motion at will.

The use of the sense of touch is contemporaneous with the use of vision. The sensation of touch, also, is not recognised at first as proceeding from external objects, but is, perhaps, only perceived as an inward sensation, and as a check to movement. Now the hand-the most important organ of touch-is one of the objects which are seen at once; and since it has the property of great mobility in space, the eye will very soon see the hand moving and touching, and many sensations caused by it will be simultaneously perceived. When the hand touches an object, the

eye sees not only the object, but the hand itself also; and when the hand is in motion we perceive simultaneously the inward sensation of movement of the muscles, that is, the existence of the sensation of touch, and, by means of our sense of sight, the visible motion of the hand and the object.

The simultaneous occurrence of the sensations of touch and sight gradually leads to the impression that an object perceived by both senses is external to ourselves. To this step of knowledge there belongs, of course, a logical conclusion, the existence of which remains a problem of mental life, but which is carried out unconsciously, and certainly only formed by degrees. It consists in this, that if the two sensations of touch and sight always take place simultaneously, they must have one and the same cause, and, therefore, that the object which is seen and touched must be one and the

same.

Still one such logical conclusion is not sufficient to enable us to recognise objects as external to ourselves. A second follows, which certainly may appear very scientific, but is not so, since it is formed unconsciously.

The two simultaneous sensations of touch and sight are two sensations of dissimilar quality, which have their nervous centres in different parts of the brain. If the cause of the sensations be found within the organs which perceive them, then it must be present at the same time in those of touch and sight, which are both different in constitution. That is to say, it must be a double one. However, according to the first logical conclusion, the cause is single, not double; therefore it is not an internal, but an external cause.

The simultaneous action of the sensations of touch and sight is, in fact, for the human mind an important source of knowledge in the external world. Yet we must not on this account conclude that touch alone, without the assistance of sight, as in the case of persons born blind, cannot lead to knowledge. It is probable that the sense of touch alone might enable us to distinguish our own body and external objects sooner than vision. For the act of touching our body with our hand calls forth a double sensation of touch, one through the hand and the other through the part of the skin touched; touching an external object causes only a single sensation of touch through the tactile organ. Upon this physiological basis, an idea of the external world might be formed, although it would be difficult to analyse such a mental act so as to arrive at a simple logical conclusion.

If once the idea is formed that the object touched belongs to the external world, the education of the sense of sight will take place rapidly. The simultaneous and coincident sensations of sight and touch--the simultaneous perception of the motion by the eye, and the sensation of it by the hand-the commencement of the sensation of touch as soon as the eye sees that the object is moved by the hand, all lead to the conviction that the cause of both sensations must be one and the same, and that if the sensation of touch is recognised as external to ourselves, this must also be the case with the sensation of sight.

The sensations of the senses must be distinguished from other kinds of sensations to which the body is subject, which are termed common sensations. With them may be classed especially the sensation of pain, which is

spread almost over the entire body. The characteristic distinction between these common sensations and the sensations of the senses is, that by the latter we gain knowledge of the occurrences and objects which belong to the external world, and that we refer the sensations which they produce to external objects, whilst by the former we only feel conditions of our own body.

The limit between the sensations of touch and pain may be illustrated by the following example given by Ernst Heinrich Weber. If we place the edge of a sharp knife on the skin, we feel the edge by means of our sense of touch; we perceive a sensation, and refer it to the object which has caused it. But as soon as we cut the skin with the knife, we feel pain, a feeling which we no longer refer to the cutting knife, but which we feel within ourselves, and which communicates to us the fact of a change of condition in our own body. By the sensation of pain we are neither able to recognise the object which caused it, nor its nature.

The sensations of the senses, therefore, appear to be of a higher kind than the common sensations. The general sensibility of the body is, indeed, the general ground from which the sensations of the senses also spring; but they are distinguished from it by a more complete perfection, since they are produced by the action of the forces of the external world upon delicately constructed organs on the surface of the body, and the mind is thereby brought into immediate communication with the external world.

The Qualities of the Sense of Touch-The Sense of Touch and the Organs
of Touch-The Sensitive Parts of the Skin.

We know from experience that every part of our skin
possesses a certain sensibility, and that this sensibility
varies in different parts. This property is given to the
skin by a great number of nerves which originate in the
brain and spinal cord, and extend in a tree-like form
over the body. The sensibility of any part of the body
is due to these nerves alone, for as soon as such a nerve
is lost or diseased, the part of the body supplied by it
becomes devoid of sensation.

We must, therefore, consider the skin as a sensory
organ which encloses our entire body, and is adapted to
render every part of the surface of our body sensible of
external impressions, and, indeed, of impressions of mani-
fold kinds, which arouse in us peculiar sensations, and are
inseparably connected with mental processes. It is re-
markable how various the perceptions are which we can

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