suggest that the direction of the light is also reversed, and the medals will seem to be what they really are.

17. Whenever an external object is watched rapidly changing its form, a continuous series of different pictures of the object is impressed upon the same spot of the retina.

Conversely, if a continuous series of different pictures of one object is impressed upon one part of the retina, the mind judges that they are due to a single external object, undergoing changes of form.

This is the principle of the curious toy called the thaumatrope, by the help of which, on looking through a hole, one sees images of jugglers throwing up and catching balls, or boys playing at leap-frog over one another's backs. This is managed by painting at intervals, on a disk of card, figures and jugglers in the attitudes of throwing, waiting to catch, and catching; or boys "giving a back," leaping, and coming into position after leaping. The disk is then made to rotate before an opening, so that each image shall be presented for an instant, and follow its predecessor before the impression of the latter has died away. The result is that the succession of different pictures irresistibly suggests one or more objects undergoing successive changes-the juggler seems to throw the balls, and the boys appear to jump over one another's backs.

18. When an external object is ascertained by touch to be single, the centres of its retinal images in the two eyes fall upon the centres of the yellow spots of the two eyes, when both eyes are directed towards it; but if there be two external objects, the centres

of both their images cannot fall, at the same time, upon the centres of the yellow spots.

Conversely, when the centres of two images, formed simultaneously in the two eyes, fall upon the centres of the yellow spots, the mind judges the images to be caused by a single external object; but if not, by two.

This seems to be the only admissible explanation of the facts, that an object which appears single to the touch and when viewed with one eye, also appears single when it is viewed with both eyes, though two images of it are necessarily formed; and on the other hand, that when the centres of the two images of one object do not fall on the centres of the yellow spots, both images are seen separately, and we have double vision. In squinting, the axes of the two eyes do not converge equally towards the object viewed. In consequence of this, when the centre of the image formed by one eye falls on the yellow spot, the corresponding part of that formed by the other eye does not, and double vision is the result.

19. In single vision with two eyes, the axes of the two eyes, of the movements of which the muscular sense gives an indication, cut one another at a greater angle when the object approaches, at a less angle when it goes further off.

Conversely, if without changing the position of an object, the axes of the two eyes which view it can be made to converge or diverge, the object will seem to approach or go further off.

In the instrument called the pseudoscope, mirrors or prisms are disposed in such a manner that the rays of light from a stationary object can be caused to alter the angle at which they enter the two eyes,

and so require the axes of these eyes to become more or less convergent. In the former case the object seems to approach; in the latter, to increase its distance.

20. When a body of moderate size, ascertained by touch to be solid, is viewed with both eyes, the images of it, formed by the two eyes, are necessarily different (one showing more of its right side, the other of its left side). Nevertheless, they coalesce into a common image, which gives the impression of solidity.

Conversely, if the two images of the right and left aspects of a solid body be made to fall upon the retince of the two eyes in such a way as to coalesce into a common image, they are judged by the mind to proceed from the single solid body which alone, under ordinary circumstances, is competent to produce them.

The stereoscope is constructed upon this principle. Whatever its form, it is so contrived as to throw the images of two pictures of a solid body, such as would be obtained by the right and left eye of a spectator, on to such parts of the retina of the person who uses the stereoscope as would receive these images, if they really proceeded from one solid body. The mind immediately judges them to arise from a single external solid body, and sees such a solid body in place of the two pictures.

The operation of the mind upon the sensations presented to it by the two eyes is exactly comparable to that which takes place when, on holding a marble between the finger and thumb, we at once declare it to be a single sphere (§ 4). That which is absolutely presented to the mind by the sense of touch in this case is by no means the sensation of one

spheroidal body, but two distinct sensations of two convex surfaces. That these two distinct convexities belong to one sphere, is an act of judgment, or process of unconscious reasoning, based upon many particulars of past and present experience, of which we have, at the moment, no distinct consciousness.

LESSON XI.

THE NERVOUS SYSTEM AND INNERVATION.

1. THE sensory organs are, as we have seen, the channels through which particular physical agents are enabled to excite the sensory nerves with which these organs are connected; and the activity of these nerves is evidenced by that of the central organ of the nervous system, which becomes manifest as a state of consciousness-the sensation.

We have also seen that the muscles are instruments by which a motor nerve, excited by the central organ with which it is connected, is able to produce motion.

The sensory nerves, the motor nerves, and the central organ, constitute the greater part of the nervous system, which, with its function of innervation, we must now study somewhat more closely, and as a whole.

2. The nervous apparatus consists of two sets of nerves and nerve-centres, which are intimately connected together, and yet may be conveniently studied apart. These are the cerebro-spinal system and the sympathetic system. The former consists of the cerebro-spinal axis (composed of the brain and spinal cord) and the cerebral and spinal nerves, which are connected with this axis. The latter comprises the chain of sympathetic ganglia, the nerves which they give off, and the nervous cords