THIRTEENTH MEETING. 21 



iDlindness arose from the complete or partial paralysis of one of the 

 three. It was met with as total and partial inability to distinguish 

 <;olours, the causes being congenital and hereditary defect ; severe 

 illness or injury, ])articularly to the spine or head ; and excessive 

 smoking and drinking. The action of light, in the normal individual, 

 on the thiree nerves is to produce the sensation of white light, under 

 vfhich the various colours had their proper effect on the organs of 

 perception, but a coloui'-blind person saw greens and reds as greys. 

 All nations exhibited a certain percentage of colour-blindness, even 

 the Indians, among whom some researches on this subject had 

 recently been made. From 2 to 13 per cent, was the usual propor- 

 tion. Colour-blindness generally existed side by side with a great 

 keenness of perception as to light and shade, so that some engravers 

 had been actually more capable owing to these personal conditions. 

 As a rule colour-blind people were unconscious of their defective 

 vision in this particular. Describing the tests for colour-blindness, 

 Dr. Ryerson said that Dr. Stilling invented a method of working 

 letters in worsted of various colours on a velvet ground. The simplest 

 and best test was, perhaps, that of Dr. Thompson, of Philadelphia, 

 which consisted of a row of different coloured skeins of worsted 

 hanging from a rod, skeins of diffei-ent colours being placed in the 

 hands of the person suspected of colour-blindness, with the request to 

 match them with those on the stick. It v^as most important that 

 railway employes should be periodically and strictly examined with 

 reference to this inability to see colour. They were frequently only 

 able to judge of colour by intensity of light, and when this ability was 

 interfered with by fog or steam the effects might be most disastrous. 

 He thought reform was needed in the arrangements of the railway 

 •comjjanies in this respect. A verdict of manslaughter should be re- 

 turned against railway companies in case of accident through neglect 

 in ascertaining whether their men could or could not judge of a 

 danger signal ; and he thought the Department of Railways and 

 ■Canals ought to take the matter up. A proper system of examination 

 for colour-blindness might be satisfactorily conducted by divisional 

 superintendents. He was not seeking to put the companies in tlie 

 hands of an army of doctors. 



Discussion ensued, in the course of which the fact was brought out 

 that researches in colour-blindness began about 100 years ago, the 



