188 Br. W. Pole on Colour- Blindness. 



is not so artificial as it at first sight appears. For mathe- 

 matical purposes the first form of the theory is the most 

 convenient. 



17. Applying the solution of § 11 to a moving dielectric 

 (constant scalar permeability and capacity) we see that the 

 velocity of light ought on the present theory to be altered by 

 the full value of the velocity of the medium. Thus the theory 

 requires some modification to explain Michelson's results, &c. 

 The last section at once suggests a possible form of such 

 modification, viz. that the theory of ' Electromag.' is true for 

 the interior of material atoms but not for the sether in their 

 immediate vicinity. In 'Electromag.' the individual atoms 

 of matter were not taken into account. To meet the present 

 theory on the fines of ' Electromag.' it would be necessary 

 to make some modification in the fundamental assumptions. 



Hobart, April 8, 1893. 



XVII. Further Data on Colour-Blindness, — No. IV. 

 By Dr. William Pole, F.R.S* 



Hering' s Explanation of the defect. 



EVERYBODY who has had to do with colour-blindness 

 has heard of the connexion with it of the eminent 

 physiologist, Professor Ewald Hering, of Prague ; but it is 

 remarkable how little is known here of his actual writings. 

 Between 1872 and 1874 he laid before the Imperial Academy 

 of Sciences of Vienna six communications on the vision of 

 light and colours, which were afterwards published sepa- 

 rately tj and have now become classical in the literature of 

 the subject. In 1879 I published in ' Nature 3 a set of 

 articles giving an abstract of the work, and this abstract 

 has formed, I believe, the only direct account in the English 

 language of Hering' s labours in the department of colour- 

 vision, although his writings thereon have been legion. 



This work, however, does not refer to colour-blindness, 

 except in a few fines promising a future special treatment 

 of it. The promise was fulfilled in 1880 by an Essay entitled 

 " Zur Erklarung der Farbenblindheit, ans der Theorie der 

 Gegenfarben," published at Prague in a scientific serial 

 called i Lotos/ Some reprints were circulated, of which I 

 was fortunate enough to get one ; but it is very scarce, and 

 is probably little known in England except to the few who 

 interest themselves specially in these foreign scientific views. 



* Communicated by the Author. 



t Zur Lehre vom Lichtsinne. Zweiter Abdruck. Wien, 1878. 





