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in the skin certain organs of temperature, which have their own nerves and are more strongly developed in some parts of the skin than in others. For it has been proved that the nerve-trunks themselves have not the faculty of producing a feeling of heat, if heat is directly applied to them. For instance, a sensitive nerve runs directly under the skin of the elbow on the bone, which causes great pain if struck. Now if we place the elbow in hot water we experience a sensation of heat in the part immersed, not in the whole arm, although the nerve extends throughout the arm and hand. We experience, however, a dull feeling of pain in the whole arm if the water is too hot. Thus the nerve-trunk is irritated by heat, which irritation, however, does not create a sensation of heat, but pain. If the elbow is placed in ice-cold water this sensation of pain is just the same, proving that the nerve-trunk can neither feel warmth nor cold. The sensation of pain which occurs in this case is another reason why the elbow should be so sensitive to a great and injurious degree of heat.

We must, therefore, assume that the nerves of the skin possess certain organs of temperature which are adapted to produce an excitement of the nerve by heat. Such organs, however, have not yet been discovered, at least none are recognised as such. It may be that the tactile corpuscles as well as producing tactile impressions may assist our sense of temperature; but nothing certain can be said on this point. Weber has discovered the interesting fact that warm bodies appear lighter than cold bodies. If a cold coin is placed upon the forehead of some person whose eyes are shut and then upon the same spot two

warm coins, the weight would seem to him the same, whilst he could distinguish cold weights correctly. A connection seems, then, to exist between the sense of temperature and the sense of touch, but it has not yet been scientifically examined.

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PART II.

THE SENSE OF SIGHT.

CHAPTER I.

Formation of the Eye-Refraction of the Light-rays in a Lens-Path of the Light-rays in the Eye.

WHILST we can only become acquainted with the presence of bodies through the organ of touch, when we bring the surface of our skin in direct contact with them, we can perceive through the eye, bodies at a greater distance, by means of what we call light. It almost seems as if no limit could be set to the passage of light through space. We receive rays of light from stars in the remotest spheres of the universe at an incalculable distance, and are thus informed of their presence. Indeed we are now enabled, by means of spectrum analysis, to determine their chemical composition. Moreover, to whatever distance a ray of light penetrates through space, as long as it possesses a certain intensity, it is able to produce an impression upon our organs of sight, and thus opens out to us a region of knowledge, the limits of which are far wider than those which enclose the domain of the other senses.

Physics teach us that light is transmitted by the

Ether, a substance of extraordinary tenuity, which extends throughout the universe, penetrates all substances, light. exists also in empty space, and that it is produced by vibrations of the ether of extraordinary rapidity. As these vibrations reach the interior of the eye through its transparent organs, they produce in us a sensation of light, and by means of the wonderful formation of the eye, we are not only able to receive the impressions of light emitted by bodies, merely as such, but also to perceive their form, size and nature.

Let us now proceed to study the most important parts of the eye.

Fig. 7 represents a horizontal section of the right eye, after Helmholtz, which will explain to us all the important parts of this organ, which nature has endued with such wonderful delicacy. We here see the eye-ball surrounded by a hard membrane, m n, called the sclerotic coat, which in living persons appears as the so-called white of the eye, in the anterior segment between the eyelids. The membrane is rather thick and strong, and provides the eye with a sufficient protection from external dangers; it is not transparent but perfectly translucent in a strong light. At the outer edge of the eye it passes into the cornea, h h, which is almost equally thick, but is distinguished by its glass-like transparency. This membrane rises and thickens in the middle like a watch glass, and also forms the transparent covering to the eye, through which the light passes into the interior.

Now in the interior there follows upon the sclerotic coat a far thinner and more delicate membrane, coloured deep black, g, called the choroid, because it contains a great number of blood-veins. Its black colour, however, pro

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ceeds from a considerable number of black pigment-cells, which are arranged like a mosaic upon its inner surface. This membrane joins the iris, pp, which is visible in every living eye, and which contains a dark opening, the pupil. Through this opening the light passes into the sclerotic coat, white

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Wh. cornea. transfor

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PP. iris, determines

i. retina.

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humour

L. crystalline lens.
G.vitreous humour

Fig. 7.

interior of the eyeball and reaches its posterior wall. The black colouring of the choroid is a great assistance to the eye's vision. Were it absent, the light, which falls upon the background of the eye would be reflected as from any other bright surface; a number of rays of light would thus be distributed irregularly in the eye, and

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