A THEORY OF COLOUR VISION 1 



By R. A. HOUSTOUN, M.A., Ph.D., D.Sc. 

 Lecturer on Physical Optics in the University of Glasgow 



I. The eye is sensitive only to a limited range of wave-lengths 

 in the spectrum, from 4000 A.U. at the beginning of the violet 

 to 7600 A.U. at the end of the red. It is not equally sensitive 

 to all the wave-lengths lying within this range, but there is a 

 maximum on both sides of which the sensitiveness falls away 

 rapidly. If the spectrum is a very bright one, this maximum 

 lies in the yellow, but if the brightness is decreased it moves 

 towards the green. 



What is the cause of this selective action of the eye ? It 

 may be ascribed to the ultra-violet and infra-red radiations 

 being absorbed by the substance of the eye, i.e. the aqueous 

 and vitreous humours and crystalline lens, before they reach 

 the retina. But this explanation is at best only a partial one. 

 It has been shown by experiment that the near ultra-violet 

 and near infra-red are not absorbed by the substance of the 

 eye, and thus, as Helmholtz points out, the selective action 

 must be ascribed to something in the retina itself. 



The selective action of the eye is analogous to phenomena 

 occurring in many other regions of physics. For example, if 

 we have two tuning-forks of the same pitch, and the first of 

 these is sounded and held near the second, the sound waves 

 strike the second and set it into vibration. There is then said 

 to be resonance between the two forks. For resonance to take 

 place in this case it is necessary that the pitches of the two 

 forks should be very accurately adjusted to equality. The 

 second fork thus exercises a selective action on the sound 

 waves ; if their pitch coincides with its own, it absorbs energy 

 from them and is set into vibration, but, if their pitch is different 

 from its own, it remains at rest. 



1 A non-mathematical account of a theory which has already been described in 

 the Proc. Roy. Soc. A. 92, p. 424, 1916. 



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