The examiner may be dissatisfied with the colour of the blue; let us, therefore, compare an examination of a normal-sighted person with that of an ordinary dichromic. The normal-sighted person will name every one of the colours with ease and certainty, with perhaps the exception of the blue, with which he is in some doubt. Here is the result of an examination of an ordinary dichromic: he called the yellow, green; the green, red; the signal green, no colour; the blue, blue; the purple, green; red A, no colour or light; red B, green. It will be noticed that the only colour that he has correctly named is the blue. We can try him again and again, and though he will mistake all the other colours he will always name the blue correctly. The examiner will have learnt from this several important facts. He will see that the colour-blind are really guided by their sensations of colour, and that it is not simply a matter of guessing. The more an examiner has practical experience of colour-blindness, the more will he recognise the fact that the colour-blind are guided by their sensations of colour. He will notice that the dichromic has readily recognised the blue which was scarcely apparent to him (the examiner), and therefore cannot have overlooked as a matter of carelessness colours which are much more apparent to the normal-sighted. The blue is a valuable colour for other reasons, for though it is not a colour on which I reject candidates, anyone miscalling it must be very carefully examined before he is passed. The trichromic generally call this blue, green. If we wish to obtain a purer blue, we can do so by combining the blue or the purple with the signal green.