190 Messrs. Magnusson and Stevens on Visual 



similar to the mode of excitation of a nerve by the alter- 

 nating electrical current, one might reasonably expect the same 

 law to hold for both phenomena. Our work shows, however, 

 that such an expectation is not justified. Probably this 

 divergence from Nernst's law is due to the previously 

 noted greater brilliancy of the light sensations produced by 

 currents of frequencies near 25 cycles. Below 15 cycles the 

 light pulsates in a succession of flashes over the whole field. 

 The quivering network appearing between 20 and 40 cycles 

 is of greater brilliancy at 25 and 30 cycles. With increasing- 

 frequency the meshes become smaller and change into steady 

 glow above 70 cycles. It is more difficult to determine the 

 threshold when the light comes as a phosphorescent glow 

 than for the pulsating flashes at the lower frequencies. With 

 the frequency above 100 cycles the light sensations remain 

 faint even from a field largely in excess of the threshold 

 value, while for frequencies below 40 cycles the intensity of 

 the sensations rapidly increase with the strength of field 

 applied. 



Coil. — In the experiments described in this paper a coil 

 consisting of five sections connected in series was used. The 

 separate sections were kept in position by means of ropes 

 passed through eyelets, shown in figs. 1 and 2 (PI. III.). 



In fig. 1, four sections are shown in position forming a 

 solenoid. The fifth section has been removed and placed 

 nearby so as to show the elliptical cross-section. 



In fig. 2, the five sections are shown with the observer in 

 the position occupied while taking observations. 



Data on Coil. 



Length (five sections) 57*0 cm. (22*4 in.). 



Majoraxis 23*8 „ (9*37in.). 



Minoraxis 2P3 „ ( 8*37 in.). 



Thickness of coil 1*14 „ ( # 45 in.). 



Area of cross-section (inside) 398*2 sq. (61*8 sq. in.). 



No. of turns in series (five sections) 1075. 



Size of wire #12 B & S, d.c.c. copper. 



Insulation Bakelite. 



Resistance of coil at 0° C 3*92 ohms. 



Inductance of coil , *09 henry. 



Reactance of coil for 60 cycles 34*0 ohms. 



Strength of Field. — Considering the coil as equivalent to a 

 solenoid of equal length and circular cross-section of equal 

 area, the maximum strength of field, H, per sq. cm., is given 



