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LIX. r lhe Eye as an Electrical Organ. By W. M. Thornton, 

 D.Sc, D.Eng., Professor of Electrical Engineering, 

 Armstrong College, Newcastle-on-Tyne* '. 



1. (~\N the electromagnetic theory of light the amplitude 

 V^ of a progressive wave in a partially conducting 

 medium is reduced by dissipation of energy as heat. The 

 optical media of the eye are typical conducting dielectrics, 

 and the following notes are a consideration of (1) the in- 

 fluence of absorption on vision, (2) the reticulation of the 

 optic nerve on the inner surface of the retina, (3) the least 

 electrical current which can be detected by the eye as light, 

 from the point of view of the electromagnetic theory. Apart 

 from the physiological question of the degree in which 

 sensation is proportional to stimulus, and considering only 

 the energy of the wave, the reduction of light in intensity 

 between entering the cornea and reaching the retina can be 

 found by the expressions given by Maxwell f and Heaviside f 

 for the decrease of amplitude of waves in a conducting 

 dielectric. 



In a distance r they are reduced in the ratio e~ ar , where 



1-41 9T 



[{*+©T-tf 



in which A is the wave-length, n the frequency, and v the 

 velocity of the wave in the medium of specific resistance p. 

 At high frequencies, such as those of light, this reduces to 



a =27rv/p. 



In the vitreous humour for example, with p = 83xl0 9 

 c.G.s. units, and with the velocity of the wave, calculated 

 from the refractive index, equal to 2*3 X 10 10 cm. a second, 

 a. is found to be 1*7, and the reduction of amplitude in 

 1*43 cm., the thickness of the vitreous humour in the human 

 eye, is to *088 of that of the incident wave. The energy of 

 the wave, which is proportional to the square of the ampli- 

 tude, is therefore *0077 of the initial value. 



The following table gives the thickness and resistivity of 

 the various media, the former for the human eye, the latter 

 for ox eyes, and the reduction of amplitude in each part. 



* Communicated by the Author. 

 + Electricity and Magnetism, vol. ii. § 798. 



\ Electrical Papers, vol. ii. p. 422. See also Lodge, Phil. Mag. April 

 3899," On Opacity." 



