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Prof. J. Joly. 



sensations: the blue and the red. Hence, although it is certain that 

 " green " quanta reach his cone, they, as such, find no sensational response 

 in his brain. He is blind to them as stimulants of green sensation. The 

 " green " quanta effect such stimulation of red and blue sensation as takes 

 place in the normal eye ; but there is a large residual number of them 

 ■which are lost to him. The total luminosity of the spectrum is less for him 

 than for normal vision, as is well seen in Abney's curves of photopic 

 luminosity. 



The red-blind or proteranojDC possesses a spectral range of colour vision 

 identical with the green and blue of normal colour sight. The maximum 

 height, as given in tliese curves, is a very little less. The blue curve is 

 normal. Hence we conclude that he possesses three-fibre and four-fibre 

 vision. All that has been said above respecting the eye possessing six 

 fibres applies to the eye possessing a total of seven fibres. Tlie loss of red 

 vision involves a considerable deduction from the total luminosity of the 

 spectrum, as Abney's curves show. 



The scotopic or rod-vision of the dichromate is normal. ■ It is the result of 

 a one-fibre stimulus. 



The monochromate's case is interesting. Apparently, he possesses but one 

 fibre leading from the cone. His foveal vision is rod-vision. The normal 

 achromatic scotopic curve and the monochromate's curve shown in Konig's 

 figure are identical. It is to be recalled tliat this curve is also that of the 

 light absorption of the visual purple and of its rate of bleaching. The 

 monochromate possesses no more than twilight vision. 



Cases of so-called blue-blindness are rare, and have generally been patho- 

 logical in character, as is the case in subjects investigated by Konig. Little 

 seems known respecting tliis abnormality. An eye with this defect would 

 possess a total of five fibres, according to the theory under discussion. 



(12) In the foregoing pages, and in those yet to come, the unit light 

 stimulus discharged by a single visual fibre is frequently referred to. It 

 represents a very small quantity of energy. Its actual amount is unknown. 

 It must not be confused with the quantum which plays the part merely of 

 the finger on the trigger. This minute quantity of energy discharged into 

 the cerebral cortex evokes our unit of luminous sensation. This is a 

 constant of deep interest to us, and one whicli dominates our psychology, 

 and, indeed, our very existence as intelligent beings. 



I propose to designate it a photon, using tlie English plural, photons. 

 Symbolically, the letter <^ will be assigned to it. 



(13) The question naturally arises : — Are we to suppose nine fibres quite 

 unco-ordinated in the cone, or are the fibres fused or grouped in bundles so 



