. Un texte-livre de physiologie animale, avec des chapitres d'introduction sur la biologie générale et un traitement complet de la reproduction ... La Physiologie, comparatif. La VISION. 587 Vers le haut ou vers le bas, vers la droite ou la gauche, il peut déterminer les limites du champ visuel d'une surface plane. Le champ visuel pour les deux yeux mesure environ 180° dans le méridien horizontal ; pour un oeil sur 145° ; et dans le méridien vertical 100°. Imperfections de la perception visuelle, - nous pouvons maintenant examiner certains défauts que nous connaissons d'exister par l'utilisation de notre raison- ing pouvoirs dans la perception mentale nous sous forme d'objets dans le champ visuel :

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. A text-book of animal physiology, with introductory chapters on general biology and a full treatment of reproduction ... Physiology, Comparative. VISION. 587 up or down, to the right or the left, he may ascertain the limits of the visual field for a plane surface. The visual field for both eyes measures about 180° in the horizontal meridian; for one eye about 145°; and in the vertical meridian 100°. Imperfections of Visual Perceptions, —We may now consider some defects which we know to exist by the use of our reason- ing powers in the mental perception we form of objects in the visual field: 1. Irradiation.—It is easy to notice that a white spot on a dark ground appears larger than a dark spot of equal size on a white ground. This has been spoken of as the result of. Fig. 428.—Illustrates irradiation. The white patch in the dark ground seems larger than the dark one in the light ground (after Bernstein). irradiation—a sort of overflow of sensation, though whether to be referred to the retina or to the brain-areas concerned is uncertain. 3. Contrast.—When a white strip of paper is laid between two black ones, the center of the white strip is not so bright as its edges, from contrast; and experiments illustrating the same principle may be made with colored paper. This law of con- trast is very wide in its application, and will be referred to later. 3. The Blind-Spot.—It might be supposed at first that one should perceive gaps in the field of vision on account of the blind-spot; but, when it is remembered that to see black we must have a definite sensation, and that the mind places objects lying on opposite sides of the spot close together, the reason that this defect in structure, if such it really be, is practically inoperative, becomes clearer. It is to be remembered that the image of an object (see Fig. 432) never falls on the blind-spot in both eyes; and, moreover, this area lies outside of that of greatest acuteness (macula lutea), on which images

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