Page:Popular Science Monthly Volume 9.djvu/712

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that, no matter under what conditions it may be compelled, by the higher law of mutual repulsion, to live, it will mould its own organism into harmony with those conditions, and thus continue its existence; and this, whether it is required to adopt a more perfect or a less perfect form.

But what it actually is, is no criterion of what it is capable of becoming, and the locality in which it is found is no evidence that it is best adapted to such a locality. These data only prove that in the final balance of forces to which it is subjected it was assigned such a degree of development and such a habitat.

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THE retina is the point where the physical process of vision passes into the physiological process. Until it impinges upon the retina, the light which penetrates the eye has only undergone physical changes, consisting chiefly in refraction, the last perceptible result of which is the production of the image upon the retina. From this point the process passes from our immediate observation, and the difficulty of discovering its character increases at each step. The image upon the retina is reversed, and yet we see every object in the field of vision upright. This is the result of the experience, which we have acquired from childhood, in the exercise of the organ of sight. The point A (Fig. 1), which is on the right, is imprinted upon the left portion of the retina, and we, therefore, know by experience that a

PSM V09 D712 Structure of the human eye.jpg
Fig. 1.

ray, coming from the right, must strike the left portion of the retina; and because we always imagine the objects we see to be external to ourselves, we must do so by unconsciously following the line a A, through the optical centre k.. In this manner the eye projects a uniform field of vision, which is obtained by drawing, from every point of the retina outward, straight lines through the optical centre of the eye, which lines will terminate upon a convex surface.

This is really the manner in which the eye interprets, in all cases,

  1. From "The Five Senses of Man," No. XXI. of the "International Scientific Series."