4()4 DR WILLIAM POLE ON THE PRESENT STATE OF 



In 1890 Professor Schuster experimented on 72 persons, 67 of whom required (in 

 converting a unit of a certain green into yellow) proportions of red varying from about 

 0*78 to 1*3 ; but four of the others used only proportions varying from about O'l to 0'36, 

 while the fifth required 2 "80. 



Some of the observers have tried to explain the physiological cause of these varia- 

 tions. Von Kkies and Frey, for example, say that the most obvious assumption is, 

 varying degrees of excitability of the sensitive element, whatever it may be ; but they 

 (•(insider it more probable, that the cause is to be sought for in an absorption by the 

 pigment of the yellow spot of the retina. Hering supports the latter view, and has 

 devoted a large portion of his brochure to an explanation of its action. Helmholtz 

 doubts the sufficiency of this, but admits that further investigation is advisable. 



It is be}"ond my province to discuss these physiological details ; but since it is now 

 established that the human eye, in ordinary normal vision, is subject to such remarkable 

 variations of impressibility by the red and green rays, it is certainly reasonable to assume 

 that, although they produce different colour-sensations in the colour-blind, these same rays 

 should still produce variable intensities of impressions upon them. We have seen how 

 slight variations in the relations between the wave-lengths, and the excitation, will pro- 

 duce the dichromic variations ; and as we now know that large variations of precisely the 

 same kind are common in normal vision, the special difficulty of accounting for them, in 

 colour-blindness, disappears. 



IV. 



FURTHER GENERAL STATEMENTS AND OPINIONS. 



1 have now endeavoured to elucidate the most important specific questions connected 

 with the Phenomena of Dichromic Vision ; but there are other matters of a more general 

 character, which have formed, from time to time, subjects of discussion, and which 

 it is desirable to mention, in order to show the views held by competent authorities 

 thereon. 



It is not to be expected that, considering the difficult and controverted nature of the 

 subject, we should find entire unanimity in the existing views ; but we shall be able to 

 trace that there has been a, gradual progress towards agreement, as knowledge of the 

 facts has extended ; we can form an intelligent comparison between differing opinions ; 

 and we may discover cases where the advance of knowledge has set aside hypotheses 

 or assumptions which formerly appeared reasonable and attractive. 



For the matters treated of here, it will be convenient to take the various writers in 

 chronological order. 



