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Proceedings of the American Academy of Arts and Sciences.

VOL. XLII. No. 16. JANUARY, 1907.

CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY OF THE MUSEUM OF COMPARATIVE ZOOLOGY AT HARVARD COLLEGE, E. L. MARK, DIRECTOR. — No. 185.

AN EXPERIMENTAL STUDY OF THE IMAGEFORMING POWERS OF VARIOUS TYPES OF EYES.

BY LEON J. COLE.

CONTRIBUTIONS FROM THE ZOÖLOGICAL LABORATORY OF THE MUSEUM OF COMPARATIVE ZOOLOGY AT HARVARD COLLEGE. E. L. MARK, DIRECTOR. — No. 185.

AN EXPERIMENTAL STUDY OF THE IMAGE-FORMING POWERS OF VARIOUS TYPES OF EYES.

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Die wirkliche Naturwissenschaft begann damit, dass man, anstatt über das Wesen der Schwerkraft zu fabuliren, die näheren Umstände der Bewegung des fallenden Steines, des Pendels u. s. f. genau bestimmte und möglichst genau und einfach beschrieb. In der Biologie, speziell in Bezug auf die uns hier interessirenden mechanischen Lichtwirkungen kann die Aufgabe des Forschers auch nur darin bestehen, die durch das Licht ausgelösten thierischen Bewegungen ihrer Abhängigkeit nach näher zu bestimmen und zu beschreiben." (Loeb, '90, p. 20.)

I. INTRODUCTION.

THE structure of eyes has been long and carefully studied, both as to gross anatomy and finest histological detail. This is especially true

of the human eye, but the eyes of other vertebrates, and of the invertebrates as well, have received a large share of attention. The ability of various eyes to form more or less accurate images of external objects has been for the most part inferred from the application of physical laws to the knowledge of their structure. In the case of man, however, there is less need for inference, since his common experience in seeing is a constant demonstration of the ability of his eyes to form images. Nevertheless, even here certain factors which add to the efficiency of the eye as a visual organ, but are in no way concerned with the actual physical formation of the image on the retina, must be taken into account. For example, judgments based upon experience involving other elements than mere image-formation, such as the mental superposition of the images of the two eyes, giving a stereoscopic effect, the action of the ciliary muscles and other mechanisms for accommodation, and the influence of other senses, especially that of touch. The filling out of the blind spot of the retina by the mind is a good example of the influence of experience upon the interpretation of the actual sense impression. It is not unlikely that the eye of a baby is capable of forming practically as good an image as that of an adult; but the baby lacks the experience and training of the adult which would enable it to form proper judgments, and consequently it appears to have little if any conception of distances and space relations in general.

The images formed by other eyes than the human can also in a measure be studied from direct observation. Thus the image of distant objects may be seen upon the retina of the freshly removed eye of an albino rabbit, since the absence of pigment leaves the posterior portion of the eyeball semi-transparent, and the relaxed eye is accommodated to distant vision. Or, the posterior portion of the eyeball of a pigmented eye, such as that of a frog, may be cut away, and by aid of a microscope the image may be projected so that it can be observed directly, or thrown upon a screen. This has likewise been accomplished with the eyes of some invertebrates; thus Exner ('91) succeeded in taking a remarkable photograph through the eye of a fire-fly, while Parker (95) demonstrated empirically that the compound eyes of Astacus form a single image rather than a number of separate images. Again, in the eyes of many vertebrates the decolorizing effect of light upon the visual purple, after protracted exposure, may be seen upon the retina,

1 The multiple photographs obtained through the corneal facets of insect eyes do not represent the retinal image formed by these eyes, since in obtaining such photographs only one portion of the dioptric apparatus is used.

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