The values of the standard colours in different parts of the spectrum are given on the right side of the above table, and are represented by the curves of Fig. 9, Plate II., where the left-hand curve represents the intensity of the “yellow” element, and the right-hand curve that of the “blue” element of colour as it appears to the colour blind. The appearance of the spectrum to the colour blind is as follows:-- From A to E the colour is pure “yellow,” very faint up to D, and reaching a maximum between D and E. From E to one-third beyond F towards G the colour is mixed, varying from “yellow” to “blue,” and becoming neutral or “white” at a point near F. In this part of the spectrum the total intensity, as given by the dotted line, is decidedly less than on either side of it, and near the line F, the retina close to the “yellow spot” is less sensible to light than the parts further from the axis of the eye. This peculiarity of the light near F is even more marked in the colour blind than in the ordinary eye. Beyond F the “blue” element comes to a maximum between F and G, and then diminishes towards H, the spectrum from this maximum to the end being pure “blue.” The results given above were all obtained with the light of white paper, placed in clear sunshine. I have obtained similar results when the sun was hidden, by using the light of uniformly illuminated clouds, but I do not consider these observations sufficiently free from disturbing circumstances to be employed in calculation. It is easy, however, by means of such observations, to verify the most remarkable phenomena of colour blindness, as, for instance, that the colours from red to green appear to differ only in brightness, and that the brightness may be made identical by changing the width of the slit; that the colour near F is a neutral tint, and that the eye in viewing it sees a dark spot in the direction of the axis of vision; that the colours beyond are all blue of different intensities, and that any “blue” may be combined with any “yellow” in such proportions as to form “white.” These results I have verified by the observations of another colour-blind gentleman, who did not obtain sunlight for his observations; and as I have now the means of carrying the requisite apparatus easily, I hope to meet with other colour-blind observers, and to obtain their observations under more favourable circumstances. MEASUREMENTS OF COLOUR FIELDS.