Another method of investigating the subject quantitatively which I employed was to determine the distance at which small spots of different colors could be recognized. I found in Murray Island that natives could see a red spot 2 mm. square at over 20 meters, while a blue spot of the same size was confused with black at even 2 or 3 meters. Europeans, however, also recognized red at a much greater distance than blue, and I have not at present sufficient comparative data to enable me to say that there is any marked difference between the Murray Islander and the European in this respect.
These results do not show that these islanders are blue blind, but they do show fairly conclusively that they have a certain degree of insensitiveness to this color, as compared with a European. We have, in fact, a case in which deficiency in color language is associated with a corresponding defect in color sense.
On the question of the cause of this insensitiveness there is room for differences of opinion. It is, of course, possible that the insensitiveness may be apparent only and may be merely due to lack of interest, but there is, I think, little doubt that it depends on physiological conditions of some kind.
The Murray Islander differs from the Englishman in two important respects; he is more primitive and he is more pigmented, and his insensitiveness to blue may either be a function of his primitiveness or of his pigmentation. In other words, it is possible that his insensitiveness may depend on the lack of development of some physiological substance or mechanism, which acts as the basis of the sensation blue in ourselves, or it may only depend on the fact that the retina of the Papuan is more strongly pigmented than that of the European. There is some reason to think that this latter factor is the more important. We know that the macula lutea in the retina, which contains the region of most distinct vision, is pigmented, and that as a consequence of the reddish-yellow color of its pigment, blue and green rays are more strongly absorbed than red and yellow; we have reason to believe further that the macula of dark races is more pigmented than that of ourselves.
The consequence would be that, in dark races, blue and green would be more strongly absorbed, and consequently there would be a certain degree of insensitiveness to these colors, as compared with red and yellow. In the observations made with the tintometer, the patches of color were so small that only the macula would have been stimulated. The probability that the insensitiveness to blue depended, at any rate partially, on the pigmentation of the macula lutea is increased by the fact that the natives were able to recognize blue readily on the peripheral retina.
It would, of course, be wrong to make any wide generalization on the basis of these observations. One would not be justified in directly
