of the two indicates the difference in sensitiveness for each part of the spectrum. [Illustration: FIG. 39.] Her persistency curve as calculated occupies the same position and is of about the same dimensions, when the maximum is made 100, as that of the normal eye, as it is therefore of red- and green-blind, and also of the two cases of monochromatic vision. We have in Miss W. a type of colour blindness which no present theory of colour vision accounts for without straining; and it would probably have to refer it to the seat of sensation rather than to the retina alone. [Illustration: FIG. 40. The thin line curve is the curve of luminosity for the normal eye.] The second is a case of congenital colour blindness and with no trace of disease, brought by Mr. Nettleship to the same Committee. He found that this lady, N. W., mistook blue for red, and it was with some curiosity that this case was examined. Her first examination was as to colour sense with the spectrum colours, a patch of monochromatic light being placed in juxtaposition with an equal patch of white light. At 62·5 (λ 6890) of the scale the light of the spectrum disappeared. As the slit moved along the spectrum, and the white was approximately reduced to equal luminosity, she described all the red as grey, and of the same colour as the white until 53·5 (λ 6110). At this point she said the colour was brownish compared with the white, and this hue continued to her till 48 on the scale (λ 5720), when she said the colour was “neither brown nor green, but both.” From 48 on the scale she described the colour as green, when it changed quite suddenly at 31·5 (λ 4905). From this point and in the blue she again began to see grey; the grey at this end of the spectrum, and also of the white patch, she called brownish-grey. This name must evidently have been a mental distinction, as she described the red end and the white as grey only, and not brownish-grey; and, indeed, she was tested again over that part of the spectrum, and adhered to the previous naming. It would appear to be due to low luminosity, which made the grey appear to her what she called brownish, rather than to any actual difference in hue.