Eye as an Electrical Organ. 563 



of Langley's value, that is 1*07 x 10 ~ 17 erg per cubic centi- 

 metre or 0*32 micro-erg per square centimetre per second. 

 The ratio of the visible to total energy can scarcely be the 

 same at the very low intensities. Retaining it in the absence 

 of more reliable data, the energy in the visible light is 

 *067 micro-erg per square cm. per second. It is then reduced 

 by absorption in the eye in the ratio "0023, giving 1*5 x 10" 4 

 erg, or with a velocity of 2*3 x 10 10 centimetres a second, 

 6*5 x 10" 15 erg per cubic centimetre of space at the retina. 



The average energy in unit volume of light-wave in non- 

 magnetic media is 47rr, where i is the root-mean-square value 

 of the current per square centimetre. The current corre- 

 sponding to the energy in the visible part of the spectrum 

 is then 



; = (6-5x10- 15 /4tt)*, 



that is 23 micro-amperes per square centimetre at right 

 angles to the wave front. 



Since there is no skin effect, this is also the current- 

 density in the nerve fibres. The current in a fibre of *0004 cm. 

 diameter would be 2*87 X 10~ 14 ampere. This, then, appears 

 to be about the least electrical current in a nerve fibre which 

 can produce the sensation of light. 



The current, when viewing white clouds in full sunlight, 

 with one's back to the sun, is about 7*0 x 10" 12 ampere in 

 the fibres. 



Taking the value of 200 ohms per centimetre cube as the 

 resistivity of nerve, the energy absorbed per centimetre 

 length corresponding to a current of 2*87 xlO - , 14 ampere 

 is 1*31 x 10" 18 watt, or 1*31 X 10~ n erg per centimetre of 

 fibre per second. If each nerve fibre conveys a separate 

 stimulus, this is what may be called the least specific stimulus 

 required in the mental process of vision ; the actual length of 

 fibre in the retinal image is a small fraction of a centimetre. 



Rayleigh * has suggested that the least power required for 

 hearing is not very different from that of least vision. From 

 Rayleigh's figures for sound, Lodge f finds this power to 

 be 6 micro-ergs per second per square cm. Comparing 

 this with the energy entering the eye at least visibility, 

 obtained above from Langley's values, viz. 0'32 micro-erg 

 per second per square cm., the eye would appear to be able to 

 detect about one twentieth of the energy required for the least 

 perception of sound. 



* ' Sound,' Article 384, footnote. 



t Jour. iDst. Elec. Engineers, vol. xxvii. p. 931. 



2P 2 



