VI. _Other tests for colour-blindness._--I have three other tests for colour-blindness: the Classification Test, the Pocket Test, and the Colour-perception Spectrometer. I have also devised an instrument for estimating the exact amount of red, at different wave-lengths, which is necessary to neutralise the complementary in different persons. 1. _The Classification Test._--(_a_) _Description._--This test consists of 4 test colours and 180 confusion colours; 150 coloured wools, 10 skeins of silk, 10 small squares of coloured cardboard, and 10 small squares of coloured glass. The whole series of colours is represented. In addition, there are a large number of colours which have been chosen by colour-blind persons as matching the test colours. The test colours are Orange, Violet, Red, and Blue-green, labelled I, II, III, and IV respectively. The colours are chosen with the view of presenting as much difficulty as possible to the colour-blind, and as little as possible to the normal-sighted. The colour-blind find especial difficulty in matching or naming a colour lying at the junction of two of their colours. As the normal-sighted often find difficulty in saying which colour predominates in a blue-green, so do the tetrachromic with their purple-green, or the trichromic with their red-green. A colour-blind person may, however, match a colour correctly which corresponds to the centre of one of his colours. In addition to choosing those colours for tests which are particularly liable to be mistaken for other colours by the colour-blind, I have used coloured materials of different kinds--wools, silks, glass, and cards--so as to force the colour-blind to judge by colour, and not by saturation or luminosity. (See Fig. 5.) [Illustration: Fig. 5.] (_b_) _Method of examination._--The candidate should be given the four test colours, and, having named each, he should be told to select all those which are similar in colour to the test colour. He should be told to pay no attention to the fact of a colour being lighter or darker; as long as it is the same colour it should be put with the test skein. The examiner should not go through the test before the candidate first of all, neither should one candidate be allowed to watch another making his selection. A shrewd colour-blind person might pass the test if he had seen a normal-sighted person go through it previously. In order to show the candidate the difference between a shade and a colour, the examiner should take one of the wools which is not a test colour--blue, for instance--and pick out four or five shades of the colour. The wools should be arranged without the knowledge of the candidate, so that a yellow or a grey is placed beside a red and the examinee asked to name its colour. At another period of the examination the yellow should be placed adjacent to a green, and the examinee again asked to name it.