192 Messrs. Mapriusson and Stevens on Visual 



o 



The above calculations are for the field strength along the 

 axis and for sine waves. Since the optic nerve lies near this 

 axis (see fig. 10), and as the field strength at the position of 

 the head is fairly uniform, it will be assumed that the field 

 strength H, as found by equation (7), applies to the area 

 between the optic nerves. 



With the observer in position, as shown in fig. 2, the eyes 



and hence the optic nerves were at a fixed distance from the 



end of the coil. This distance was measured and found to be 



as follows: — 



Observer. Distance from lower end of coil. 



M . . . . 17-0 cm. 



S . . . . 17-0 cm. 



P . . . . 14-7 cm. 



L . . . . 17-0 cm. 



These positions are indicated in fig. 4 by the dotted lines. 



Since all the observations recorded in this paper for the 

 threshold values were made with the observer and the coil 

 in the relative position shown in fig. 2, the value of H at the 

 distance of the eyes from the lower end of the coil is of chief 

 importance. 



In order to ascertain conveniently the field strength in the 

 plane of the optic nerves of each observer, in terms of the 

 amperes as given in the tables, a set of curves were drawn 

 as shown in fig. 5. 



Curve " C ,J gives H at the middle of the coil. 



Curve "MSL" gives H at 17*0 cm. from lower end of 

 the coil. 



Curve " P " gives H at 14*7 cm. from lower end of coil. 



It is readily seen that the relation between the field 

 strength and current is a constant for each observer, as 

 follows : — 



MSL 30-7 



P 30-2 



In Tables I., II., III., IV., H' is the product of the 

 respective average currents and these constants, and thus 

 gives the maximum number of lines of force per sq. cm. 

 along the axis of the coil and in the plane of the optic nerves 

 of the observer. 



Generators and Wave Forms. — For frequencies of 11 to 

 67 cycles inclusive, the current was obtained from an 

 alternator having a slotted armature and with a normal full 

 load rating of 60 K.W., 60 cycles, 1100 volts. Power was 

 supplied by a d.c. motor, belted to the generator and 

 arranged for the required range of speed-variation. The 



