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The reasons on which the Committee have based these .recommendations are set forth in the following pages.
The subject of colour-sense and its imperfections is one introductory, which is necessarily of great scientific interest: but it also has a practical importance, as it affects a definite proportion of the men who are engaged in the two great industries of railway traffic and of navigation. Amongst railway men, at least, if not also amongst sailors, a suspicion has been excited that the methods adopted for testing coloursense are not entirely trustworthy, and have had the effect of excluding some individuals from employments, the duties of which they were well qualified to discharge. On this ground alone, if on no other, it has seemed advisable to the Committee that the reasons for their recommendations should be so stated as to be intelligible, as far as possible, to all those who are interested in the matter.
Every colour, and among colours for convenience sake are included black and white, can be defined by three qualities:—1st, its hue—thus we talk of red, green, violet; 2nd, its purity, or the measure of its freedom from admixture with white—which is expressed by such terms as "deep" or "pale;" and 3rd, its brightness or luminosity—thus we say a colour is "bright," or "dark." Two colours are identical only when they can be defined as possessing the same three colour qualities, or constants as they are called, and if they differ in any one they are no longer the same. When two objects are compared together for colour, the large majority of persons will agree as to their identity or difference. Their verbal descriptions of the difference may vary slightly, but practical tests show that in reality they recognize the same variations, and hence their vision is termed normal vision. There is, however, not an inconsiderable minority, as will presently be shown, whose perception of colour differs very widely from that of the majority, and, for want of a better term, members of this minority are -called " colour-blind." By this term it is not intended to convey the idea that there is absolute insensibility of vision, or even of colour-vision, but merely that the ordinary distinction between certain colours is defective The variations in the amount of this deficiency in colour-perception are numerous, and when small, are often exceedingly difficult to classify.
We have to regard these deviations from normal vision more -from a practical than from a theoretical standpoint, and in testing for them we have to take the broad view that the colourblindness which has to be detected is that which may be dangerous to the public in the industries already mentioned.
There are some few people who fail to distinguish blue Character of from green, and others, equally few, who only see in mono- f?.lo°rchrome, but the colour-blindness which is most common, and, blmdne"therefore, most dangerous, is the so-called red-green blindness, in which there is a failure to distinguish between red and
green; that is to say, a red-green blind person will regard a certain hue of green as identical in colour with some hue of red, another of green as identical with white, and some will also fail to see red at all of another particular hue. When it is considered that on our railways white, green, and red lights are used as safety and danger signals at night, and that the same colours are not unfrequently used for a similar purpose by day, it is very obvious that to place persons who are red-green blind in positions where the colours ought to be correctly recognised may be the cause of disasters. The same objection to the employment of persons with defective colour-vision applies also to navigation, for at night the presence of a green or red light on the port or starboard side indicates the course that a vessel is taking, and if either those in charge, or on the look-out. are colour-blind, serious risks of collisions are run. Description of It is proposed to enter somewhat minutely into the the spectrum, characteristics of red-green blindness, showing how it may be divided into two species. For this purpose it is necessary to appeal to the spectrum. When a thin slice of white light falls on one or more prisms, or on what is known as a diffraction grating, it is decomposed into a parti-coloured band which we call the spectrum, the principal colours, as given by Newton, being red, orange, yellow, green, blue, indigo and violet. If the light be that from the sun innumerable black lines will be seen interrupting this series of colours, some more marked than others. It is found that these lines always occupy the same position as regards the colour in which they are situated, and hence the more pronounced ones will act to the spectrum as milestones do to a road. Different coloured rays have different lengths of undulations in the all-pervading medium which is called ether, and the wave lengths of the coloured rays which, if present, would occupy the place of the principal black lines have, notwithstanding their minuteness, been determined with extreme accuracy, and this enables the position of any particular hue of spectrum colour to be numerically fixed by a reference to the wave lengths of these lines. We have said that the principal spectrum colours are those stated above, but it must be understood that they are only fully recognized by persons possessing normal vision; for the spectrum would be described by a colour-blind person in very different terms. For instance, some red-green blind would say that the red, orange, and yellow were all yellow; red would be described as dark yellow, orange as less dark, and yellow as bright yellow, whilst the green part of the spectrum bordering on the yellow would be described as yellow diluted with white. In the pure green would be pointed out a white or grey band, and the bluegreen would be described as blue diluted with white; whilst the blue would be called light blue, and the violet dark blue (see No. 2, Plate I). Others, again, whilst similarly describing the blue and violet part of the spectrum would substitute green
for 3-ellow in the above description of red, orange, yellow, and yellow-green, the brightest red would be called dark green, and they would fail to see at all in the extreme red, the spectrum being shortened. These latter would also recognize a white or grey band, but it would be in a position rather nearer the blue of the spectrum than in the first case (see No. 3, Plate I). It is needless to say that to normal vision this white or grey band is non-existent, and whenever a person under examination sees such a band the evidence is conclusive that he is colour-blind. These differing descriptions of the spectrum show that this form of colour-blindness may be divided into two classes, which for convenience sake may be termed green- and red-blindness. Another point of difference between them is the part of the spectrum that appears brightest. To the normal eye it is the yellow, and to the green-blind it is nearly at the same place, but to the redblind it is the green. This, perhaps, may give a clue to the designation of the spectrum colours by these two classes. To the green-blind, red and yellow are the same colour, but the yellow being the brighter he looks on red as degraded or darkened yellow. On the other hand, to the red-blind green is brighter than yellow or orange, and these appear as degraded green.
Experiment has shown that every colour in nature, as seen by a normal eye, can be expressed as a mixture of three, so that normal vision is tri-chromatic. In a similar sense the more pronounced types of ordinary colour-blind vision are di-chromatic. These colour relations must be regarded as purely subjective, for enough is now known of the nature of light to exclude the possibility of a three-fold physical constitution. In the theory of Young, subsequently, and independently, brought forward and developed by Helmholtz, light is supposed to be capable of exciting three distinct primary sensations, combined in varying proportions, and dependent upon the quality of the light. As to the character of the three sensations, Young identified them with red, green, and violet; and no widely-differing choice is possible, unless upon the supposition that the primary sensations, in their purity, are quite outside the range of our experience. The yellow of the spectrum, for example, cannot be primary, for it is capable of being matched by a suitable mixture of red and green. According to this view each primary sensation is excited in some degree by almost every ray of the spectrum; but the maxima occur at different places, and the stimulation in each case diminishes in both directions, as the position of maximum is receded from.