in the true sense of the word; neither does anything of the nature of light ever start from the sun. Light has no more existence in nature outside a living body than the flavour of onions has; both are merely sensations.

If a boy throws a stone which hits you in the face, you feel a pain; but you do not say that it was a pain which left the boy’s hand and travelled through space from him to you. The stone, instead of causing pain in a sentient being, might have broken a window, or knocked down an apple. Just so, the same radiation which, when it chances to encounter an eye, produces a certain sensation, will produce a chemical decomposition if it falls upon a cabbage, an electrical effect in a selenium cell, or a heating effect in almost anything. Why, then, should it be specially identified with the sensation?

“Radiation” also includes, and is nearly synonymous with, what is often miscalled radiant heat. After what has been already indicated, I need hardly say that there is no such thing as radiant heat. The truth is that the sun or other hot body generates wave-energy in the ether at the expense of some of its own heat, and any distant substance which absorbs a portion of this energy generally (but not necessarily) acquires an equivalent quantity of heat. The result may be exactly the same as if heat left the hot body and travelled across space to the substance; but the process is different. It is like sending a sovereign to a friend by a postal order. You part with a sovereign and he receives one, but the piece of paper which goes through the post is not a sovereign. It is strictly correct to say that the sun loses heat by radiation, just as you lose a sovereign by investing it in the purchase of a postal order. But that is not the same thing as saying that the sun radiates heat.

The term “radiation” has the advantage of avoiding any suggestion of the fallacy that there is some essential difference in the nature of the ether-waves which may happen to terminate their respective careers in the production of light or heat or chemical action or something else; but it is, unfortunately, impossible in the present condition of things to use it as freely as one could wish without pedantry, and we must still often speak of light or of heat when radiation would express our meaning with greater accuracy.

Light, then—to use the term unblushingly in its objectionable but well understood sense—has the property of stimulating certain nerves which exist in many living beings, with the result that, in some unknown and probably unknowable manner, a special sensation is called into play—the sensation of luminosity. And in order that the creature may be able not only to perceive light but also to see things, that is, to appreciate the forms of external objects, it is generally provided with an optical apparatus by means of which the incident light is suitably distributed over a large number of independent sensitive elements.

In man and the higher animals the optical apparatus, or eye, consists of a stiff globular shell, having in front an opening provided with a system of lenses, and, at the back of the interior, a delicate perceptive membrane, upon which the transmitted light is received. So much of the light emitted or reflected from an external object as passes through the lenses, is distributed by them in such a manner as to form what is called an “image” upon the membrane, every elementary point of the image receiving the light which issues from a corresponding point of the object, and no other. The contrivance evidently bears a close resemblance to a photographic camera, the sensitive plate or film, upon which the picture is projected, being analogous to the perceptive membrane.

I am not going to attempt a detailed description of the human eye. It will be sufficient to point out briefly some of its principal features as indicated in the annexed diagrammatic section, Fig. 2.

Fig. 2.—Diagram of the Eye.

The opening in front of the globe is covered by a slightly protuberant transparent medium C, which is shaped like a small watch-glass, and on account of its horn-like structure has been named the cornea. The space between the cornea C and the body marked L is filled with a watery liquid A, known as the aqueous humour: this liquid with its curved surfaces constitutes a meniscus lens, convex on the outer side and concave on the inner. Then comes the biconvex crystalline lens L, an elastic gelatinous-looking solid, which is easily distorted by pressure. The convexity of this lens can be varied by the action of a surrounding muscle M M, and in this way the focus is adjusted for objects at different distances from the eye. When the muscle is relaxed and the lens in its natural condition, the curvature of its surfaces is such that a sharp image is formed of objects distant about forty feet and upwards. When by an effort of will, the muscle is contracted, the lens becomes more convex, and distinct pictures can thus be focussed of things which are only a few inches away. This process of adjustment by muscular effort is technically known as “accommodation.”

The remainder of the globe is filled with the so-called vitreous body V, which derives its name from its fancied resemblance to liquid glass: it might perhaps be more properly likened to a thin colourless jelly. The vitreous body plays a part in the refraction of the light.

The perceptive membrane, or retina R R, which lines rather more than half the interior of the eye-ball, is an exceedingly complex structure. Though its average thickness is less than ¹⁄100 inch it is known to consist of nine distinct layers, most of which are marvels of minute intricacy. Of these layers I shall notice only two, the so-called bacillary layer, which is in immediate contact with the inner coating of the eye-ball, and the fibrous layer, or layer of optic nerve fibres, which is only separated from the vitreous body by a thin protective film.

The bacillary layer (from bacillum, a wand) consists of a vast assemblage of little elongated bodies called rods and cones, which are placed side by side and set perpendicularly to the surfaces of the retina, or in other words, radially to the eye-ball. Let us try to make the arrangement clear by an illustration.

Imagine a small portion of the inner surface of the eye-ball, one-tenth of an inch square, to be magnified 2000 diameters (four million times), and let the enlarged area be represented by the floor of a room 17 feet square. Procure a quantity of cedar pencils, and set them on the floor in an upright position and very close to one another. It will be found that the number of pencils required to fill the space will be about half-a-million. To make the analogy more complete, let some of the pencils be sharpened to a long tapering point at their lower ends, the greater number remaining uncut, just as received from the manufacturers. Neglecting details which are immaterial for our present