62 Dr. W. Pole on Colour-Blindness. 



coloured, and the trichromic variety of hues will no longer 

 exist. 



This more general theory of dichromatisin appears, so far as 

 I can understand it, to amount to the view that while the two 

 dichromic hues uiust be in some way derived from the three 

 fundamental colours of normal vision, there is no necessary 

 condition as to the maimer of their derivation, or as to what 

 they shall be*. 



The author, further, in regard to his new theory, calls 

 attention to a point of considerable importance. One of the 

 greatest stones of stumbling for years past has been the 

 division, consequent on the "old" explanation, of dichromic 

 patients into the two theoretically distinct classes of " red- 

 blind "" and " green-blind." It is obvious that this division 

 naturally disappears when the old explanation is abandoned ; 

 but Helmholtz takes pains to show geometrically that his new 

 theory gives no place for such a division. And he, moreover, 

 expresses the opinion that such a division does not seem to 

 have been fully justified by observation. 



He adds, " By this it is also shown that the want of corre- 

 spondence between the absent colour of the dichromic system 

 and one of the fundamental colours found by us, does not in- 

 volve any insoluble contradiction |. 



It is not my business to offer any comments on the views 

 I have endeavoured (imperfectly I fear) to call attention to ; 

 I have only to repeat the reference to the original for fuller 

 information regarding them. But even though there should 

 be differences of opinion on minor points, there can be 

 none as to the character of the work or the position of its 

 author among the great men of the age. 



Athenaeum Club, S.W., December, 1892. 



* Only it would seem that as the dichromic eye now loses nothing', 

 but uses the whole of the three fundamentals and no more, and as 

 their sum makes normal white, then the total sum of the dichromic 

 vision must make normal white also, aud the two hues, X and Y, will 

 be complementary. — W. P. 



t On account of the great importance of these two expressions of 

 opinion, I subjoin Helrnholtz's own words (page 461): — "In unseren 

 Betrachtungen ist keinerlei Beschrankung fur die Lage der Schnitt- 

 punktes gegeben. Daher fallt bei dieser Verallgemeinerung der Theorie 

 der Dicnromasie auch die Trennung in zwei scharf getrennte Klasseu, 

 Grlmblinde und Rothblinde, weg, welche ja auch den Beobachtungen 

 gegeniiber nicht ganz gesichert erscheint. 



" Daniit ist auch nachgewiesen, dass der Mangel an Ueberstimmung 

 zwischen derfehlenden Farbe der dichromatischen Systeme und je einer 

 der yon aus gefundenen Grundfarben keinen unloslichsn Widerspruch 

 einschliesst." 



