378 Mr. H. R. Droop on 



ceeds from Professor Donders, who suggests that where one 

 of the three coiour-sensations has been wanting from birth, 

 its absence may have modified the development of the other 

 two sensations. He says: — " The retina is not an instrument 

 with three strings of which one has suddenly snapped. It is 

 a living instrument whose three differently toned strings have 

 been developed in conjunction with each other " (Grafs Archiv 

 fur Ophthalmologic, vol. xxvii. p. 212). 



This explanation is ingenious; but it assumes that all cases 

 of colour-blindness to red and green are from birth, whereas, 

 though this is usually so, there are several alleged cases of 

 acquired colour-blindness to red and green. Dr. Joy Jeffries, 

 pp.. 50-52, gives two cases, one discovered by Professor 

 Tyndall, the other by Mr. Haynes Walton; and M. Nuel has 

 also described one, Annales cle V Oculiste, 80, 82, as quoted by 

 v. Kries, Gesichtsempfindungen, p. 154. 



Moreover Professor Cohn (Deutsche medicinische Wochen- 

 schrift, 1880, No. 16, cited by von Kries, p. 158) claims 

 to have temporarily restored to normal colour- vision a person 

 affected with red-green colour-blindness from birth. If this 

 be so, there cannot well have been any such abnormal deve- 

 lopment of his other colour-sensations as Professor Donders 

 supposes. Such a development could hardly have been sud- 

 denly cured by artificial means. 



Those colour-blind persons who cannot distinguish between 

 red and green have been divided into two classes, according as 

 they can perceive rays towards the end of the spectrum or 

 are unable to do so. According to many adherents of the 

 theory of three colour-sensations, those who can perceive rays 

 at the red end want the green-colour sense, while those who 

 cannot do so want the red sense. It seems to be established 

 that rays at the green part of the spectrum do not make as 

 much impression on the so-called green-blind as on the so- 

 called red-blind ; and Professor Donders has lately ascertained 

 by careful measurements with two cases, one of so-called red- 

 blindness, and the other of so-called green-blindness, that 

 throughout the red and orange parts of the spectrum the red- 

 blind eye perceives less light than the green-blind eye, and 

 that the opposite is the case in the green portion of the spec- 

 trum (Graf's Archiv fur Ophthalmologie, vol. xxvii.; Transac- 

 tions of the International Medical Congress for 1881, vol. i. 

 p. 277). 



But there seem to me to be several serious objections to ex- 

 plaining the difference between so-called green-blind and so- 

 called red-blind by the hypothesis of three colour-sensations: — 



(A) If there are only three colour-senses, the green sense 



