tion caused by light, which takes place in the rods, passes to the optic nerve. The optic nerve ultimately conveys it to the brain, and there awakes the sensation of light.

The trunk of the optic nerve, just like any other nerve, can be excited directly without the intervention of the retina, and by every such excitement the sensation of light is produced. If, in operations upon the eye, the optic nerve is severed, the patient has no sensation of pain, but of a brilliant flash of light, upon which follows perpetual darkness. We may also excite the optic nerve by electricity, if we pass a current through the socket of the eye and the cranium, when flashes of light are always observed in the field of vision.

Investigations in the domain of nerve physiology have led to the result, that all our nerves are merely instruments for the conveyance of one and the same action-the irritation of the nerve, which passes along them, in accordance with similar laws. The nerves of touch, sight, hearing, taste and smell all agree in their nature. The optic nerve no more conveys the lightwaves to the brain, than the auditory nerve conveys the sound-waves. But both nerves at their terminations possess different kinds of apparatus, which are called the sensory organs, and by means of which they can be excited. The terminal apparatus of the optic nerve is the eye, or, more strictly speaking, the retina, which has the property of being excited by the light-waves, and of communicating this excitement to the optic nerve. The terminal apparatus of the auditory nerve is found within the inner ear, and by its vibration excites the auditory Now the action is identical in the two sensory organs as soon as it has reached the nerve, and carries

nerve.

with it no trace of a musical sound or of a ray of light. The two actions produce in our brain, however, sensations of so different a nature, because each nerve has a special centre in the brain, where it terminates. The centre of the optic nerve has peculiar properties different from those of the centre of the auditory nerve. The first only arouses the sensation of light as soon as it is set in action, the latter only produces sensations of sound. The action of both centres is quite mechanical, and therefore they can each only produce one and the same effect. From this reason light is perceived, even if no light enters the eye, as soon as the optic nerve is torn, severed, or excited by electricity, because the optic nerve communicates its excitement to its own centre, which replies with a sensation of light. The same is the case with the auditory nerve, whose centre can only produce the sensation of sound, and never anything else. under accidental circumstances, the auditory nerve had been attached to the eye, and the optic nerve to the ear, then every ray of light would produce a sound, and every sound in our ear would produce an appearance of light in our imagination; we should then see a symphony, and hear a picture.

If,

Returning to the action of the sensation of light in the retina, we may imagine it to be as follows :—

The light-waves, in a manner not yet quite understood, give rise to a process in the rods and cones of the retina which very probably consists of intermolecular motion. The rods and cones communicate this internal motion, by means of the fibrous prolongations already mentioned, to the upper layers of the retina; and thus from a particular cone, the motion reaches the nerve-fibre

H

with which it is connected. The motion then produces an excitement of the nerve, which excitement produces in the brain the sensation of light.

manner.

By a very ingenious calculation Heinrich Müller has shown that the rods and cones are the point where the perception of light takes place. For this purpose he makes use of the arborescent figure of Purkinje. If, for instance, the flame is moved to and fro (fig. 20), the figure moves also, as is shown by the construction. Now this figure is formed by the shadow of the vessels of the retina, and from the construction given above, the position of the shadow can be calculated in the following If we measure the angle a k around which the candle flame moves, and which is equal to the angle bk b', we can find the distance bb upon the retina. Now the distance of the vessel v from bb can be determined approximately, therefore the angle bvb can also be determined, which is equal to the angle c v c'. Now c and are the shadows of the vessel, which apparently moves in the field of vision from d to d. We, therefore, measure by observation the angle d k ď, which is equal to c k d, calculate from it the line cd, since we know kc, and now, in the small triangle cvc, which may be regarded as an isosceles triangle, we know the base c d and the angle c vd. This gives us the distance of the vessel v from the point where its shadow falls; and, in fact, we find that the shadow falls exactly upon the rod and cone layer.

Now, what is the nature of the action of light upon the rod and upon the shadow? With our present knowledge of the properties of light, we can imagine some possible answers. For instance, the action may be a

chemical one. This conjecture is most probable, because we know that light, by the chemical decomposition of iodide, or chloride of silver, can produce a picture on a photographic plate. We may, therefore, suppose that in the rods and cones a substance is also present, which undergoes chemical change by the action of light, andre of that, at the same time, a real substantial picture isation formed upon the retina. Of course, this picture would not be a permanent one like that of a photograph, for it disappears as soon as the light ceases to fall upon the eye, and we must suppose that the nutritive action, or circulation of the blood, always destroys it. Such a chemical picture would be able to excite the termina

tions of the optic nerve, for we know that nerves can be excited by chemical agencies. Still, all this is mere conjecture, and similarly with the possibility that the action of light on the retina may be due to electrical action, which we know takes place in nerves and muscles. In short, it still remains a problem for science to solve, what it is which gives the rods and cones the remarkable property of exciting the optic nerve by means of light.

the retina

Lee, bean

CHAPTER V.

The Colours of the Spectrum-Combination of Colours-The Three Primary Colours-Colour-blindness

RADCLIFFE

THE light which we perceive in nature is by no means of the same kind, but we distinguish a number of kinds of light which we term colours. All objects possess a certain colour, due either to the light which they reflect, or which they transmit. We speak, therefore, ordinarily of coloured light, although physics tells us that colour cannot be separated from light, as if it were like a colour which an artist lays on a picture. The kind of light which we call white, can be decomposed, by the prism, into a number of colours, which comprise all the simple colours, and which, when combined, produce all the colours which occur in nature. If a sunbeam is allowed to fall upon a prism, as is shown in fig. 27, the beam is decomposed into a number of colours, whose sum is called a spectrum.

A ray falls through the opening b, upon the prism s. If the ray continued without interruption, a bright circle d, would fall upon the opposite wall. By refraction, however, the ray is turned aside, and since the coloured rays which it contains possess different degrees of refrangibi