SIGNIFICANCE OF EYE MOVEMENTS 



553 



always made use of movements of the arm and especially of J;he eyes them- 

 selves, which have taught us the proper direction. 



As an illustration of the way visual impressions are projected outward we 

 may take Schemer's (1619) experiment, which at the same time is an interest- 

 ing demonstration of accommodation. Two needles are placed one behind the 

 other before a bright background, the one vertical and about 18 cm. from the 

 eye, and the other horizontal and about 60 cm. from the eye. Then a card con- 

 taining two small holes, whose distance from each other is less than the diameter 

 of the pupil, is held before the eye, and the other eye is closed. If one accom- 

 modates now for one needle, while the card is being held so that the line joining 

 the holes is in the same direction as the other needle, this second needle will 

 appear double. 



Suppose the eye be adjusted for the distant needle, b (Fig. 243, A), then 

 the image of the near needle a falls at a. Since each of the two holes admits 

 a beam of light from the near needle, and these two beams cannot fall in the 

 same place on the retina, two faint images are formed at the places where they 

 cross the retina. In the same way by accommodating for the near needle a we 

 get a double image of 1) (Fig. 243, B), because the rays from that needle strike 

 the retina at two places. If in the latter case one hole in the card is covered, the 

 image on the same side will disappear; for the image which is formed by cross- 

 ing (cf. Fig. 230) to the opposite side of the retina has been projected to this 

 side of the field of vision e. g., the upper image at a in the direction of b'c. 

 If, however, the same hole c be closed in the first case (Fig. 243, A), the image 

 'on the opposite side disappears; the lower image at 6' is projected not in the 

 direction c, but in the direction d. 



Movements of the eye determine the projection of our visual impressions in 

 other connections also. When one looks through a wire gauze at the window 





FIG. 243. Schemer's experiment. 



the meshes appear large and far removed from the eye, but if the eyes be 

 focused on a pencil point held close to one's near point of vision in front of 

 the gauze, the meshes appear small and near i. e., appear in the plane of the 

 fixed point or of the point where the lines of vision meet. Although the experi- 

 ment gives the same result in looking with one eye, the observer can plainly 

 feel that the eyes are strongly converged in fixing the near object. 

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