﻿134 Prof. E. Rutherford on the Retardation of the 



on the screen and to ask a red-blind to make a match in 

 luminosity with the white. The latter will have to be 

 reduced to almost darkness — a darkness, indeed, that makes 

 the match almost incredible." 



It seems incredible that such a person, when comparing 

 red and green with a Flicker photometer, would obtain the 

 same results as i£ he had normal sight. However, it 

 appears that, according to Dr. Edridge Green *, colour- 

 blindness is of two kinds. A person may be unable to dis- 

 tinguish, say, red-light by colour, but nevertheless a red 

 object may appear as luminous to him as to anyone else. On 

 the other hand, the colour-blindness may be due to the fact 

 that the eye is incapable of perceiving red light at all. A 

 colour-blind person of the first variety would presumably 

 make normal photometrical readings. A person of the second 

 class must, surely, obtain abnormal readings with, all photo- 

 meters, Flicker or otherwise. 



This is borne out by some of Professor 0. N. Rood's experi- 

 ments on flicker f. He found that those of his students who 

 were colour-blind obtained abnormal results with his Flicker 

 photometer. Indeed, he actually used the Flicker photometer 

 to investigate not only cases of colour-blindness but also the 

 difference in sensibility to light of different colours, of the eyes 

 of persons with normal sight. 



In conclusion the writer wishes to express his great in- 

 debtedness to Professor Ayrton, and also to Mr. J. M. McEwan, 

 for their assistance and for many valuable suggestions. 



XIX. Retardation of the a Particle from Radium in passing 

 through Matter. By E. Rutherford, F.R.S., Projessor 

 of Physics, McGill University, Montreal^. 



[Plate II.] 



IN previous papers § I have given an account of some ex- 

 periments which show that the a particles decrease in 

 velocity in their passage through matter. The source of rays 

 consisted of a fine wire, which had been made active by 

 exposure as the negative electrode in a vessel containing a 

 large quantity of radium emanation. 



Fifteen minutes after removal from the emanation, the 



* " The Physical Aspects of a Theory of Colour Vision," by F. W. 

 Edridge Green, M.D., British Association, 1902. 

 t American Journal of Science, 1899, p. 258. 

 X Communicated by the Author. 

 § Phil. Mag. July 1905, January and April 1906. 



