We are now in a position to carry the investigations as to luminosity a little further. When we look at small patches of light, we view the colour through the yellow spot in the eye. If, when we have matched the luminosity in the ordinary manner, we turn our eyes some 10° away from the patches, we shall find that except at one place in the green the equality in brightness no longer exists. By a little practice we can make matches of luminosity when the eyes are thus diverted. This will give us a different curve of luminosity, as the yellow spot absorption is absent, and the difference in the heights of the ordinates between the two curves will give us that absorption. Fig. 20 shows this very well; and it will be noticed that the eye is appreciably not so sensitive to the red and yellow at 10° from the axis as it is on its central area. If we measure the areas of these two curves we get the relative values of the light energy which is active on the two parts of the eye, and these we found to be as 167 to 156. The heights at which to put the maxima of the two curves were found from various considerations, and the correctness of the deductions was verified by directly comparing the intensities of two patches of white light some 10° apart, which, when looked at direct, were of equal intensities. When one was compared with the other, the eye receiving one image centrally and the other outside the yellow spot, the difference in values was closely proportional to those of the above areas. The part of this last curve showing a deficiency in red sensation is very similar to that obtained from a person who is partially colour blind. The absorption by the yellow spot derived from these measures is graphically shown in the next figure (Fig. 21). [Illustration: FIG. 20.] [Illustration: FIG. 21.] The question of the visual sensation at the “fovea centralis” (if it be admitted that this is coincident with the visual axis of the eye, as is usually accepted) may be very easily studied. When the luminosity of the spectrum is examined at five or six feet distance, by throwing the two patches on the whitened face of a small square of half-inch side, we get a result differing from both of the above. The fovea appears to be slightly more sensitive to red than the macula lutea, and is generally less sensitive to the green rays (see Fig. 20). If a star, or a distant light, be observed with the part of the retina, on which the axis of the eye falls, as is the case in ordinary vision, and then be observed with the eye slightly directed away, the difference in the colours of the light is unmistakable. (The tables giving the measured value of these curves will be found in the appendix, page 211.) [Illustration: FIG. 22.]