194 Messrs. Magnusson and Stevens on Visual 



effect of a higher maximum value, and that the effective 

 instead of maximum values should be used in making com- 

 parisons of threshold stimuli. Since in computing H and 

 H 7 the constant -y/2 was used, the values in the tables are 

 directly proportional to the effective flux values. 



Lines of Force cutting the Retina and Optic Nerve. — An 

 alternating current produces an alternating field, and hence 

 the flux H moves radially toward the axis of the coil as the 

 current increases, and in the opposite direction when the 

 current decreases. Therefore, the flux appearing in any 



area like oab in fig. 9, bounded by two radii ox and oy and 

 a nerve, represented by the line a, 6, cuts across the nerve, 

 a-b four times for each cycle. 



In order to show what part of the flux cuts the optic 

 nerve, fig. 10 was drawn showing the relative size and 

 position of the coil, the skull, the optic pathways, and field 

 of vision. In fig. 10 the coil is presented as circular. As a 

 matter of fact the coil was elliptical, the major and minor 

 axes of which were 23*8 and 21*3 cms. respectively. Since 

 the movements of the lines of force are at each point normal 

 to the ellipse, no flux will cross the major and minor axes. 

 The angles AOB and COD show the incidence of the lines 

 of force upon the right and left retinas. It is evident that 

 more lines of force cut the nasal halves of the retinae than 

 the temporal halves. It is also apparent that the optic nerves 

 lie almost parallel to the normal of the elliptic coil, and that 

 they therefore will be cut by comparatively few lines of 

 force. 



The solenoid was made light-proof by fastening strips of 

 rubber insulator over the joints between the segments of the 

 coil and by covering the top with felt. The lower end into 

 which the head of the observer was placed was closed by 

 stuffing felt wadding around the neck of the observer. 



