34-8 PRACTICAL PHYSIOLOGY. [LXXIII. 



(/.) The length of a line appears to vary according to the angle 

 and direction of certain other lines in relation to it (fig. 269). The 

 length of the two vertical lines is the same, yet one appears much 

 longer than the other. (A large number of similar illusions will be 

 found in Du Bois-Reymond?s Archiv, 1890, 

 p. 91, by F. C. Muller-Leyer, and Laska, 

 p. 326.) 



6. Imperfect Judgment of Distance. 



(a.) Close one eye, and hold the left 

 forefinger vertically in front of the other 

 eye, and try to strike it with the right 

 forefinger. On the first trial one will 

 probably fall short of the mark, and fail 

 to touch it. Close one eye, and rapidly 

 try to dip a pen into an inkstand, or put 

 a finger into the mouth of a bottle placed 



Fro. 269. To show False a ^ a convenient distance. In both cases 

 Estimate of size. one will not succeed at first. In these 



cases one loses the impressions produced 



by the convergence of the optic axes, which are important factors 

 in judging of distance. 



(b.) Hold a pencil vertically about 15 cm. from the nose, fix it 

 with both eyes, close the left eye, and then hold the right index- 

 finger vertically, so as to cover the lower part of the pencil. With 

 a sudden move try to strike the pencil with the finger. In every 

 case one misses the pencil and sweeps to the right of it. 



(c. ) Fix a wire ring about 3 inches in diameter into the end of a rod about 

 2 feet in length. Hold the rod at arm's-length, close one eye, try to put 

 into the ring a vertical process attached to a rod of similar length held in the 

 other hand. 



7. Imperfect Judgment of Direction. 



As the retina is spherical, a line beyond a certain length when 

 looked at always shows an appreciable curvature. 



(a.) Hold a straight edge just below the level of the eyes. Its 

 upper margin shows a slight concavity. 



(6.) In indirect vision the appreciation of direction is still more imperfect. 

 While leaning on a large table fix a point on the table, and then try to arrange 

 three small pieces of coloured paper in a straight line. Invariably, the papers, 

 being at a distance from the fixation-point, arid being seen by indirect vision, 

 are arranged not in a straight line, but in the arc of a circle with a long 

 radius, 



8. Perception of Size. 



Fix the centre of fig. 270 at a distance of 3 to 4 cm. from 



