562 Prof. W. M. Thornton on the 



(about 4 to 10 ya), that notwithstanding the frequency 

 approaching 10 15 a second, the skin effect does not occur. 

 That is, the current density of the electrical current forming 

 the light-wave, which being transverse traverses the fibres 

 longitudinally, is uniform over their cross-section. In the 

 calculation of this the resistivity of the fibres was taken to be, 

 as a lower limit, 20 ohms per cm. cube. The value given 

 by Waller* is 200 ohms per cm. cube for muscle and nerve; 

 the previous value was chosen to cover any possible increase 

 of conductivity in non-medullated fibres. If the inner layer 

 of nerve fibre were continuous and of the same thickness as 

 the fibres, it would not be opaque to electric radiation at the 

 frequency of light, unless by selective absorption. The object 

 of the reticulation cannot therefore be only to let the light 

 through to the rods and cones. 



From observations given later, it can be shown that the 

 energy absorbed in the faintest visible light is not sufficient 

 to account for a rise of temperature in the fibres of more 

 than 10 ~ 8 degree C. a second ; and since the eye is able to 

 follow rapid flicker, it seems improbable that the visual 

 stimulus can be in any way thermal in a medium maintained 

 at blood-heat. It is more probable, and it is suggested here, 

 that the stimulus caused by the electrical currents in the 

 light- wave incident on the fibres may be contributory to vision 

 by acting as a continuous "messenger" to the brain, or as 

 the vibrator in a coherer system keeping sensitive the contact 

 at the synapse between the retina and the rods and cones. 



3. An approximate estimate of the least current which 

 can be detected by the eye as light, may be made by con- 

 sidering the distance at which the sun would cease to be 

 visible. The mean energy reaching the earth's surface in 

 full sunlight was found by the late Prof. S. P. Langley to 

 be 4*3 X 10~ 5 erg per cubic centimetre. In a letter from 

 him, shortly before his death, he estimates the energy in the 

 visible part of the spectrum from the curves of luminositv 

 to be 21 per cent, of the whole. The stellar magnitude of 

 the sun according to Pickering is —25' 5. The mean of 

 Wollaston's, Zollner's, and Bond's values is —26*4. Thus 

 with the former value the sun has 4 x 10 12 the intensity of a 

 star of the 6th magnitude, the highest visible by the unaided 

 eye, with the latter value 9'1 x 10 12 . Taking the former as 

 probably the more accurate, the sun would cease to be visible 

 at 2 x 10 6 its present distance f. 



The energy reaching the earth would then be 0'25 x 10 ~ 12 



* A. D. Waller, ' Signs of Life.' 



t I am indebted to Prof. R. A. Sampson for the astronomical data. 



