BY JOHN THOMSON, M.B. 43 



Dr. Jay Jeffries, of Boston, U.S.A., in his work on Colour- 

 Blindness is more assertive and declares that railway and 

 marine accidents have occurred from it and cites instances. 



Someone has suggested the impossibility of getting at the 

 truth, of subsequently proving or disproving colour-blindness, 

 owing to the individual responsible for the catastrophe being 

 lost with his ship or wrecked with his train. 



It has been proposed to alter the lights and employ those 

 not likely to be mistaken by partially or completely colour-blind 

 people; colours from the extremes of the spectrum, red, orange 

 or yellow, and blue, blue-violet, and violet. 



Orange and yellow are out of the question, for they would 

 readily be confounded with other lights which are not signals 

 chiefly in the vicinity of towns, just as white was— and it has 

 been discarded as the "clear" or safety signal on the railways. 

 Red alone remains ; it is a saturated and brilliant colour ; a red 

 glass transmits about 10 per cent, of the luminosity of the lamp 

 behind it; in red theory and practice agree; experiment and 

 experience force its use. 



Blue, blue-violet and violet are impracticable owing 

 to lack of brilliancy and indistinctness when viewed from a 

 distance ; glasses of these hues only transmit from 2 to 4 per 

 cent, of the light behind. 



The red and green must remair ; they cannot be altered to 

 suit the individual ; he must be selected to recognise them, and 

 in the selection " the greatest severity should be observed, or, 

 in other words, the least defect in the sense of colour should be 

 a sufficient ground for rejection." (Regulations for the 



management of State railways in Sweden). 



Testing for Oolour-Blindness. 



The test adopted is the one recommended in the report of 

 the committee on colour vision, i.e., Holmgren's, introduced in 

 1878, the well known coloured wool-matching test. A complete 

 set of Holmgren's wools runs into many dozens of skeins, and 

 many of these are diificult to procure, and are unnecessary. 

 Professor Thomson, of Philadelphia, suggested a modification of 

 Holmgren's methods, and this was adopted and successfully by 

 the Pennsylvania and other American railways. I have used it 

 for some twelve years ; it is the scheme I have already referred 

 to and on which my table is based, and I have confidence in it. 

 But practically it still remains Holmgren's, and Holmgren's has 

 been adopted by almost every country — European and American 



