344 Report of the Committee on Colour-Vision. 



Evidence of Dr. George Lindsay Johnson. 



The Chairman : You are aware, perhaps, Dr. Johnson, that 

 this Committee is investigating- the general subject of colour- 

 blindness. We gather that you have given your attention to 

 the subject, and should be glad of any information you can give 

 the Committee as to your experience of practical testing by 

 various methods ? — I am acquainted with most methods. I have 

 used the spectroscope, and recently a simple form of Captain 

 Abney's method. I have also had a little experience with 

 Donders' method. Some of my testing has been with the 

 spectroscope with a graduated circle, in which yon read off the 

 point where the spectrum appears to the patient to end. It is 

 only with red-blind and green-blind cases I have had much 

 practical acquaintance ; although I have had one violet-blind. 

 In using the spectroscope, I ask the patient to fix the point where 

 the spectrum appears to end, and read it off on the scale, to see 

 if I can get an improvement benefiting the patients who are 

 red-blind, and in order to practically measure the improvement 

 under special treatment. I may say with coloured wools or 

 ordinary reflected colours I do not think it is so easy to ascer- 

 tain whether patients make a definite improvement as by 

 measuring with a Vernier's scale, with which the exact limit to 

 which the red end extends can be made out. 



Some patients find a difficulty in fixing the exact limit to 

 which the red extends ; but, as a rule, with intelligent patients, 

 thejr can fix it pretty definitely. 



The source of light I have hitherto employed has been a 

 candle at Moorfields Hospital, and a paraffin lamp at home. 



I find there is a variation according to the light used, and to 

 prevent error on this account I have always used with the same 

 patient the same source of light. Sky-light gives different 

 results to candle-light, and also with regard to the fields of 

 vision. 



In taking- fields of vision I generally use Dr. Priestley Smith's 

 perimeter, which I have modified somewhat myself. I am not 

 quite satisfied with the dead or pigment colours, but adapt to the 

 perimeter an instrument for taking fields of vision, which I have 

 had constructed for use with a 2-candle power incandescent 

 electric-light. 



[The Witness handed in a diagram illustrating the apparatus 

 referred to, and explained with reference to its use.] 



Question. — Does your experience go to prove that the spectrum 

 colours all disappear at the same angle as Landholt holds, or do 

 you find a difference in the disappearance according to the 

 intensity of the light employed ? — Yes. I am certain the differ- 

 ence is in accordance with the intensity of the fight. I have not 

 got figures at present with regard to the exact point where the 

 colours stop with the spectrum, and do not think my figures 

 would be of much use, unless interpreted by Fraunhofer's lines. 



