476 DR WILLIAM POLE ON THE PRESENT STATE OF 



NOTES. 



On the alleged Danger with Coloured Railway -Signals. 



I have hitherto avoided mentioning this much agitated question, because it is really a 

 consideration more for common-sense than for science. If colours must be used as signals, 

 surely it should not require elaborate treatises or extensive investigations to convince us 

 that the men who have to distinguish them should be able to judge of colours properly. 



At the same time, there are a few points on which light may be thrown by a know- 

 ledge of the true phenomena. One is the explanation of the remarkable fact, that 

 although there must have been multitudes of colour-blind railway-servants, yet, during 

 the whole time railways have been in existence, no accident has ever been shown to arise 

 from mistaking the signal-lamp colours. We know now that although these men may 

 have had no idea whatever of the two normal sensations called red and green, yet the 

 two signal-lights may have presented to them other features of contrast, sufficiently 

 obvious to guide them in the distinction. 



I had once a striking illustration of this fact when attending an examination of 

 engine-drivers. Noticing one man who had shown himself, by the ordinary tests, hope- 

 lessly colour-blind, but who seemed otherwise intelligent and capable, I asked leave to 

 put a few questions to him ; and, taking samples of actual red and green signal-glasses, I 

 said — 



Q. (Holding the glasses to a light in the dark room) Here are two coloured glasses, — we won't 



trouble ourselves about their names. Do they appear to you alike or different ? 

 A. Quite different. 



Q. Can you distinguish one from the other, and can you recollect them ? 

 A. Yes. 



Q. I will call this A, and the other Z. Can you remember this distinction ? 

 A. Yes. 



I then tried him, by exhibiting them many times singly or together, and asking 

 which they were ? He always answered correctly and promptly except once — as he 

 explained, by an accidental inadvertence, and not from any doubt. Indeed, he seemed to 

 consider (as I myself should have done) that the idea of their appearing similar was a 

 very foolish one. 



But as there is no certainty that such a distinction should always exist, the fact ought 

 not to militate against the systematic exclusion of colour-blind men, as a matter of 

 reasonable precaution. 



There are many effectual ways of doing all that is essential in testing the vision ; for 

 where red-green blindness really exists, it is so patent that it must show itself on the 

 simplest intelligent examination. But, unfortunately, the old theoretical obstinacy has 

 intruded itself here to make the process difficult and troublesome; for Holmgren has much 



