The science of life : fully illustrated in tone and line and including many diagrams

BOOK 1

and oozing out through fine pores at the edges of the cornea ; by means of this slow, steady stream, food and oxygen are brought to the cells of the lens and adjacent structures and their waste products are washed away.

The eye, then, is provided with a combination of two smoothly-curved bodies, the cornea and the lens, by means of which light can be focused on to the retina. By

far the larger part in image formation is

Fig. 49. A rod (below) and a cone (above) from the retina, highly magnified.

The sensitive end is to the left.

played by the cornea, which is responsible for about two-thirds of the bending of rays of light. ‘The importance of the lens lies in the fact that it is elastic. By means of muscle fibres which pull on it the convexity of the lens can be altered; the optical system is therefore adjustable, and in this way we can focus our eyes on objects of various distances. Sometimes this apparatus does not work as well as it should. The muscles may not pull properly, or the eyeball itself may be slightly out of shape, so that the image is not crisply focused. In other and more familiar words we may be short-sighted, or long-sighted, or astigmatic. Butsuch defects can be easily remedied by putting extra lenses into the optical system—by an eyeglass or a properly adjusted pair of spectacles.

The sense-cells of the retina are of two kinds, called rods and cones. ‘The appearance of these two elements is shown in Fig. 49. Each has at one end a striped structure, longer and more slender in the rods than in the cones, which is presumably the seat of stimulation, and each tapers away into a long nerve-fibre at the other end. It used to be thought that these cells were directly sensitive to light, but it has recently been shown that this view is incorrect. Light

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a foot from the face.

THE SCIENCE OF LIFE

Fig. 50. Close the left eye and look fixedly at the cross with the right, holding the page ten inches or

By moving it nearer or farther a distance can be found at which the circle disappears, its image falling on the blind spot.

CHAPTER 3

acts upon them in a curious and roundabout way. The matrix in between the retinal cells contains chemical substances which are decomposed by light, in much the same way as the silver compounds of a photographic plate are decomposed by light, and it is the substances resulting from this decomposition, not the light itself, that stimulate the cells. The rods and cones are not sensitive to light at all; they are chemical senseorgans like tastecells or smell-cells, and just as sound is converted into touch-stimuli in the ear, so light is convertedinto chemical stimuli in the eye. Sight is, in fact, an extreme utilization of chemical response. It is the brain which interprets the impulses of the retinal fibres as form and colour.

We have seen that the great variety of touch-sensations are in reality only blends of one or two elementary kinds of feeling. A similar analysis can be applied to coloursensations. It is known that we can give

rise to any desired colour-sensation by projecting

into the eyeball the correct

The blind spot.

mixture of red, green, and violet light. Probably there are three different lightsensitive substances in the retina, decomposed by red, green, and violet light respectively, and their decomposition-products give rise to the three elementary coloursensations. ‘he process of blending these three sensations into a compound colour such as yellow or brown or white, occurs in the brain and is analogous to the mental blending of different notes into a chord. The retinal cells communicate by means