CHAPTER X.
VISION AND OPTICAL ILLUSIONS—THE EYE DESCRIBED—ACCOMMODATION OF THE EYE—CHROMATIC ABERRATION—SPINNING TOPS.
The eye is an optical instrument that may be compared with those constructed by physicists themselves; the media of which it is composed have surfaces like those which enter into the construction of optical instruments. It was Kepler who at the end of the eighteenth century discovered the passage of light into the eye. Soon after the discovery of the inner chamber he found that the eye realized the conditions that Porta had combined to obtain the reflection of external objects.
Fig. 95.—Structure of the eye.
We will now briefly state that the coats of this organ are constituted of a fibrous membrane, T (fig. 95), termed sclerotic, which is opaque, except in the anterior portion of the eye, where it forms the transparent cornea. The crystalline, C, enshrined behind the cornea, is the convergent lens of the inner chamber; it is covered with a transparent membrane, or capsule, and is bathed in two fluids, the aqueous humour, between the crystalline humour and the cornea, and the vitreous body, a gelatinous humour lodged between the crystalline and the back of the eye. The image of exterior objects which is produced by the passage of light through these refracting surfaces, is received by a nervous membrane, the retina, B, formed by an expansion of the optic nerve, N. We must also mention the choroid, a membrane lined with a dark pigment, which absorbs the light, and prevents interior reflections, and in front of the crystalline lens, a curtain with an opening, H, called the iris, which gives to the eyes their colour of blue, grey, or black. The opening in the centre of the iris is called the pupil.
The penetration of light through the surfaces of the eye is easily demonstrated. An object throws divergent rays on the cornea, a part penetrates into the eye and falls upon the retina, leaving a perfectly retained image of the object. Magendie has proved in the following manner the truth of this mathematical deduction. The eye of a rabbit is very similar to an albino’s; that is to say, the choroid contains no black pigment, but a transparent matter, and when placed before a brilliant object, the image can be seen inverted on the retina. The experiment succeeds also with the eye of a sheep or a cow, if the sclerotic has been lessened. The optic centre of the eye is the point where the secondary axes cross; the optic axis passes through the geometrical axis of the organ, and directs itself spontaneously towards the point that attracts the eye.
Fig. 96.—Diagram of mode of vision.