2. _Trichromic vision._--These persons see three distinct colours in the spectrum--red, green and violet. They describe the region intermediate between red and green; that is to say, the orange and the yellow as red-green, and blue as violet-green. It will be seen, therefore, that their chief difficulty is distinguishing yellows and blues. A yellow, for instance, which is situated next to a green will be called red, and the same yellow when adjacent to a red will be called green. There are various degrees of trichromic vision, varying from those who are little better than dichromic to those who are tetrachromic. The trichromic rarely find any difficulty with their three main colours--red, green and violet. These cases have been described under the name of anomalous trichromatics. This name is one which has been given to those persons who in making a match between a yellow corresponding to the sodium flame and a mixture of thallium-green and lithium-red make a mixture which is different to that of the normal.[9] A man who puts too much green in the mixture is called a green anomaly; whilst a man who puts too much red in the mixture is called a red anomaly. The red anomaly is only a trichromic with shortening of the red end of the spectrum, and this may be as extensive as in any case of dichromic vision. I have, however, described trichromic cases which had all the symptoms attributed to the anomalous trichromatics, but they were not anomalous trichromatics, as they made an absolutely normal match.[10] [9] _Proceedings of the Royal Society_, vol. B 76, 1905. [10] _Transactions of the Ophthalmological Society_, 1907, p. 255. _Proceedings of the Royal Society_, vol. B 82, 1910. LECTURE II _Delivered on February 3rd_ THE DETECTION OF COLOUR-BLINDNESS FROM A PRACTICAL POINT OF VIEW I. _Object of a test for colour-blindness._--Tests for colour-blindness are of two kinds; namely, those which are used for the purpose of ascertaining the special phenomena of colour-blindness, and those which are employed when the inquiry is made for some practical purpose. As with visual acuity, it is necessary to fix an arbitrary standard. As we do not wish to exclude a greater number than is absolutely necessary, the object of the test should be to exclude dangerous persons and dangerous persons only. These persons may have other duties to perform which do not require them to possess a perfect colour-sense. I should, however, like to see those persons who are specially qualified for a certain position, occupy it, for instance, men who have to keep a look-out on our most important ships being selected because of their accurate colour-vision and visual acuity. I do not mean that I would select only those men and reject the others, but that I should like to see a second object of a test, namely, to select those who are specially efficient so that the Captain might know on whom to rely in conditions of exceptional difficulty.