II. _The requirements of a test for colour-blindness._--A test for colour-blindness, when it is to be employed for some definite and specific purpose, as, for instance, excluding dangerous persons from certain callings, should be such as to show definitely that the persons rejected are dangerous. It is very useful to demonstrate to the men and their fellows that a rejected candidate is dangerous. The colleagues of a rejected candidate would refuse to risk their lives with a man who before their eyes called a red light, green. I was expressing these views when a superintendent of a railway company, who is using my lamp, told me that he had adopted this method with great satisfaction to himself and to the men. A man, for instance, who has been working twenty years on the railway has been rejected for colour-blindness. He has complained bitterly to the superintendent, at the same time declaring emphatically that he is normal-sighted. The superintendent has replied, “You know red?”--“Yes.” “You know green?”--“Yes.” “You will therefore agree that if you call green, red; or red, green, you ought to be rejected. Bring two or three of the other men in with you and I will test you.” The man has readily agreed to this. The superintendent has then tested him by asking him to name various coloured objects in the room, and knowing by experience exactly the coloured objects which are miscalled by the colour-blind readily exposes his defect. It is noteworthy that on some occasions a colour-blind man has been tested by another person in the same room without making any of the mistakes which he subsequently made, because none but coloured objects which he could readily recognise were shown to him. This is an example of the necessity of a practical knowledge of colour-blindness in an examiner. On account of the arrangement of signals by sea and land, it is necessary that persons employed in the marine and railway services should be able to recognise and distinguish between the standard red, green, and white lights in all conditions in which they are likely to be placed. An engine-driver or sailor has to name a coloured light when he sees it, not to match it. He has to say to himself, “This is a red light, therefore there is danger”; and this is practically the same as if he made the observation out loud. Therefore, from the very commencement we have colour-names introduced, and it is impossible to exclude them. The engine-driver is told that red is a “danger” signal, green a “caution” signal, and white an “all right” signal. Therefore, it is absolutely necessary that he should know the meaning of these colour-names. A test should be such as to make it impossible for the examinee to be coached through it. This is one of the most important requirements of a test for colour-blindness and one that is rarely fulfilled. Nearly every one of the tests in general use fail on this account.