master was so incredulous), and said that he had something the matter with his eyes when a child.

I have also used squares painted with various mixtures of colours in order to test the seven-unit, with the same result. It is obvious that practically this class is of little importance, as they are able to recognize all the differences in colour which are perceptible to other persons, and they are able to recognize differences which are not perceptible to others. It is probably on this account that systems of colour nomenclature have found little favour, and there is not a definite system in general use. It will be noticed in Fig. 1 that the blue encroaches on the green of the normal-sighted. A seven-unit person would therefore call a colour a blue which a normal-sighted person would call a green. This is just the opposite to the mistakes of the colour-blind, who include a portion of blue in their green.

CHAPTER XI.

THE QUALITATIVE AND QUANTITATIVE ESTIMATION OF DEFECTS OF COLOUR-PERCEPTION.

BEFORE discussing the methods which should be employed to ascertain the degree of colour-defect of a colour-blind person, it is necessary to consider in detail the causes of colour-blindness. These may be ranged under four heads. I. Colour-defects due to absorption of certain rays by the media of the eye.

II. Colour-defects due to non-excitability of the visual substance or optic nerve-fibres by rays of light of a certain wave-length.

III. Colour-defects due to pathological conditions.

IV. Colour-defects due to imperfection of the colour-perceiving centre.

Each of the above subdivisions requires to be considered separately. On examining a case of colour-blindness, we must ascertain under which of the above heads the defects must be classed. We may meet with any one of the above conditions alone or combined, and it is this want of classification which has made the study of colourblindness so difficult.

An examination with the spectrum should first be made to ascertain the psycho-physical perception of the individual examined, and whether there is shortening of

the spectrum for bright light, or light of diminished intensity.

With regard to the first class, namely, colour-defects due to absorption of certain rays by the media of the eye. I think that the importance of defects due to this cause has been very much exaggerated. The absorption is so slight in comparison with the quantity of light which enters the eye, that in the absence of any other colourdefect there would scarcely be any alteration in the appearance of colours. Again, the colour-perceiving centre would become less sensitive to impressions of the predominant colour, and more sensitive to those of its complementary. I. have carefully looked for a case in which the whole colour-defect might be put down to absorption by the media of the eye, and have not been able to find one. The case of Mulready will, however, serve as an example of defects of colour-perception due to this cause. It is well known that the pictures which he painted in his later years are too blue, and in order to be seen to the best advantage they have to be looked at through a yellow glass. The cause of this defect was the yellow tint which the crystalline lenses acquire with age.

The degree of colour-defect due to absorption may be ascertained quantitatively. The result, however, will be only relative to that of the examiner. For instance, Mulready relatively to most persons saw too much yellow in nature, whilst the same persons taking Mulready as a standard saw too much blue. In order to ascertain quantitatively the degree of the defect we must first exclude colour-blindness due to defective psycho-physical perception, and colour-blindness due to a shortened spectrum, and then test the individual with an apparatus for mixing spectral colours in definite proportions. The appearance of definite mixtures should be noted. A certain amount

of blue being found to neutralize a definite amount of yellow for the examiner, it should be noted whether more, or less blue is required for the examinee, and the relative proportions of each colour. The yellow and blue must in every case correspond to definite points of the spectrum scale. The intensity of the colour may be regulated by increasing or diminishing the intensity of the source of light. In the same way the proportions of red and green necessary to make yellow, and the proportions of green and violet necessary to make blue should be ascertained. The proportions of the different complementaries necessary to make white should be recorded.

There is not much to be said about Class III., namely, colour-defects due to pathological conditions. These rarely exist apart from other diseases of the eye, and are fully considered in treatises on these diseases. The remainder of this chapter will be devoted to considering Classes II. and IV., namely, colour-blindness due to shortening of the spectrum, and colour-blindness due to defective psychophysical perception.

Methods which are employed in order to ascertain the nature of the colour-perception of the colour-blind must be separated from, and considered entirely apart from those which are employed in order to determine whether colour-blindness be present or not for some definite practical object. The length of time which must be spent over one case, in order that it may be subsequently used as a basis for classification, is generally very great. The results obtained by the following methods agree in all particulars.

A. Information afforded by the Colour-blind themselves. We may obtain very valuable information from the colour-blind, if their remarks be taken down without question. It is absolutely necessary that no leading ques

tions be asked. A colour-blind person who talks about a greenish purple probably belongs to the four-unit class, whilst another who talked about a reddish green would probably belong to the three or two-unit class.

The following questions should be asked:

1. How long have you found difficulty in distinguishing colours?

2. What colours do you confuse?

3. What effect has gaslight upon colours?

4. Does distance make any difference in the appearance of colours?

5. Are you fond of colours? What is your favourite colour?

6. Does the ordinary classification of colours appear to you correct?

7. Give examples of objects which appear of a decided colour.

8. What colours appear the most definite?

9. Have you tried to cultivate a knowledge of colours, and, if so, with what result?

10. Have you any relative who cannot distinguish colours?

He should then be asked to name a large number of coloured objects. This method of examination is very useful as a preliminary, because a colour-blind person will name colours in accordance with his psycho-physical perception of colour, and we shall thus obtain a very good idea of the class to which he belongs. Many writers on colourblindness have stated that naming colours is a useless and misleading method of examination, because the colour-blind must use the conventional colour-names, and use them at random. But this argument is a fallacy, because the colourblind do not name colours at random, but in accordance with their ideas of colour. I will make this clear with an