The Eye as an Electrical Organ. 



561 



The resistances were measured by the Kohlrausch bridge- 

 telephone method on freshly killed, though cold, eyes. 



The high conductivity of the humours, greater than that of 

 blood, is remarkable. 



Medium. 



Thickness r. 



9- 



Mean. 



a. 



ar. 



Eeduction 

 ratio. 



Cornea 



'115 cm. 



455 



455 



03 



•0345 



•968 



Aqueous humour. 



•36 



92-114 



103 



1-37 



•495 



•610 



Crystalline lens... 



•39 



650-750 



700 



020 



•078 



•925 



Vitreous humour. 



1-43 



80-87 



83 



» 



243 



•088 



The total reduction of amplitude is the product of all the 

 figures in the last column and is equal to "048 ; the energy 

 is therefore *0023 of: that of the incident light. 



The reduction of intensity is independent of frequency, 

 that is, of colour. The limitation of vision at the blue end 

 of the spectrum is not then due to simple absorption of the 

 kind considered, but either to selective absorption — probably 

 in the cornea — or to photo-chemical inactivity of the visual 

 yellow and purple in ultra-violet light. 



The greater part of the incident energy is absorbed before 

 reaching the vitreous humour, but the rate of diminution in 

 the latter is so great, that in the case of exposure to very 

 strong light, injury is lessened. The chief function of the 

 vitreous humour, from this point of view, is to act as an 

 absorbing screen protecting the retina from possible over- 

 exposure. 



Although absorption would appear to reduce the sensitive- 

 ness of the eye by its presence, the structure of the retina 

 can by it be much more delicate without risk of injury. 



2. Light falling on the retina encounters first the reticulated 

 optic nerve. The reason for the fine subdivision of this is 

 not fully known. The size of the fibres is, however, such that 

 they are peculiarly well suited, as a bolometer, to receive 

 the incident energy. 



The distribution of alternating current in the cross-section 

 of a cylindrical conductor is not uniform when the frequency 

 exceeds a certain value, which depends upon the conductivity 

 and diameter. Since the current density is then greater at 

 the circumference, this is known as the " skin effect " ; it 

 reduces the effective current in the conductor. 



The diameter of the nerve fibres of the retina is such 

 Phil. Mag. S. 6. Vol. 20. No. 118. Oct. 1910. 2 P 



