two colours be viewed through a glass which is opaque to the rays occupying the shortened portion of the spectrum, they will appear identical in colour and shade.

1. Confusion of Red and Green.-This variety of colourblindness has attracted more attention than any other. The confusion of red with green is essentially a mistake of persons belonging to the class of the two-unit. For all practical purposes it is a mistake which is diagnostic of the two-unit. As it may be met with in other conditions, it is necessary that these should be mentioned. The three-unit are perfectly cognizant of red and green as colours, and therefore would not mistake the one for the other. If they were asked to name some hundreds of red and green objects they would give the correct answer in each case; but the following are the conditions in which they might mistake the one for the other. They recognize yellow on account of its superior luminosity; but if this luminosity be reduced to that of the red and green they will be compelled to judge of the result by the colour. This is at once called a reddish green, or a greenish red, according to the apparent predominance of one of these colours. If the colour correspond exactly to the modi· fied unit red-green, the three-unit will not be able to distinguish which colour predominates, and are therefore very likely to call a reddish light, green, or a greenish light, red. The mistakes made are very much increased by shortening of the red end of the spectrum. Shortening of the red end of the spectrum, when met with in cases of the four, five, or six-unit, is not likely to cause confusion of red and green. It could only cause a red to be mistaken for a green under certain circumstances. The most penetrating rays of the spectrum are those on the extreme left, at the commencement of the red; therefore, if we had a colour which was red by virtue of these rays, but reflected a certain

number of green rays, it would appear green to a person with a spectrum shortened at the red end because the red rays would not be perceived. This is the actual constitution of many red lights when seen at a distance. Therefore there are persons who find to their surprise that though they can distinguish a red from a green easily enough when close to the colours, at a distance they either do not see the red at all, or it has changed to a green. On walking towards the red they find that it suddenly springs into view through the orange and red rays, which had previously been obstructed, reaching the eye. The reason why the red is not mistaken for the green when the observer is close to the colours, is that there are few, if any, coloured substances which reflect only the green and the extreme red. Nearly all red substances reflect the orange and the other red rays in such predominance as to cause the colour to be perceived as red, even if a portion of the red rays, which are reflected, be deducted from the total composition of the colour.

2. Confusion of Green and Brown.-This mistake is practically diagnostic of the three-unit at least. The three-unit regard brown (which is a deep shade of yellow) as a reddish green or greenish red, according to the portion of the spectrum to which the brown corresponds. A large number of browns which present no trace of green to the normal-sighted are called greens by the three-unit. Under the name of brown many colours are classed by the normalsighted. Thus there are orange-browns, red-browns, yellow-browns, and purple-browns. The four-unit can always distinguish the yellow-brown, which is a deep shade of yellow, from green; but this is the colour which presents especial difficulty to the three-unit.

3. Confusion of Blue and Green.—This is the diagnostic sign of the four-unit; a Cambridge blue, such as 40 and

45 of the Pocket Test, being the colour which is especially confused with green. An ordinary two-unit does not mistake this colour for green, but clearly distinguishes between the two. A two-unit, however, with a neutral band will, if the neutral band extend over this portion of the blue, confuse blue and green. For an instance of this see the case of Mr. Rix. The five-unit and the three-unit find a certain difficulty with blue and green, but not in the very marked degree that the four-unit do. It will be noticed that the blue is called green, and not the green blue, in accordance with the fact that the green has encroached upon the blue and not the blue upon the green. In the Board of Trade Report on Colour Tests for 1889, pale blue was called green in 249 cases, but pale green was called blue in only 6 cases.

4. Confusion of Colours other than those mentioned above. The mistakes which I have mentioned above are those with which we most commonly meet. An epitome of the mistakes made by each class of the colour-blind will be found at the end of the chapter on the "Classification of the Colour-blind," to which I will refer the reader, as these mistakes are not diagnostic in character, and therefore do not call for special comment.

CHAPTER XVI.

PREVALENCE OF COLOUR-BLINDNESS.

AT present it is not possible to say accurately to what extent the various degrees of colour-blindness exist, for the reason that nearly all the examinations have been made with Holmgren's test; and I have shown in other chapters that a red-green blind may only make such mistakes as would class him with those who have defective chromatic perception. Again, the four-unit colour-blind may make mistakes which would class them with the completely colour-blind. Again, there are many colourblind persons who would escape detection by Holmgren's test. The percentage of colour-blindness by this method is about 4 per cent. The methods pursued by Dr. Wilson give more accurate results. He gives the percentage of colour-blindness at about 5 per cent. He divides the colour-blind into the following classes-those who confound blue with green; those who confound brown with green; and those who confound red with green. The following is the percentage he obtained from the examination of 1154 persons. Those who confound blue with green, 2.2 per cent., or 1 in 46; those who confound brown with green, 16 per cent., or 1 in 60; those who confound red with green, 1.8 per cent., or 1 in 55. This gives a percentage of the colour-blind, including all classes of 56 per cent., or 1 in 177.

From the examinations which I have made, I have obtained very similar results; the percentage of the twounit being about 2; the percentage of the three-unit being about 1.5; and the percentage of the four-unit being about 3. As the five-unit and the four-unit are, for all practical purposes, normal-sighted, they need not be included in the percentage. This makes the percentage of the colour-blind about 3.5 per cent., and this is near enough for all practical purposes.

About one-fifth of the educated male persons whom I have examined have had diminished colour-perception. The large majority of them belong to the class of the fiveunit, and it is hardly fair to class them with the colourblind at all. Women have a very much better colourperception than men. For instance, in testing thirty ladies I only found one with diminished colour-perception. This lady belonged to the class of the five-unit. By the ordinarily used tests about one woman in two thousand is found to be colour-blind.

Colour-blindness, in my experience, is rather more common in the uneducated than the educated. With one exception, I have not found any particular class of individuals in whom colour-blindness is more frequent than in other classes. The one exception to which I refer is the class of musicians. I have found more colourblind persons amongst musicians than in any other class or profession.