"My late wife was partially colour-blind. She could not see at a distance the colour of a red railway-signal. She could not see the red coat of a soldier at two hundred yards distance; a bright orange-colour display of the aurora borealis was to her white. Yet when the objects were near she could distinguish colours perfectly. The red geraniums near my window she could see as well as I could; but those a hundred yards off were lost in the green. She could choose and compare coloured silks or worsted, and could paint well, and was a remarkably good colourist. And yet, like 'A. W. T.,' had she been put to drive a railway train she would have driven it to de

struction.

"Yours truly,

"Richmond, Surrey, January 24, 1890."

"G. P.

2. Confusion of Violet and Black.-Confusion of violet and black indicates shortening of the violet end of the spectrum. If the violet be only indistinguishable from black at a distance, it is probable that the shortening is not very great. Thus Z. A., a two-unit with shortening of the violet end of the spectrum, remarked to me that he could not distinguish a deep violet dress from black. It is obvious that the violet confused with black must be very dark, because substances of a lighter colour reflect or transmit other rays than the violet; whereas substances of a dark colour generally reflect only those rays which are peculiar to the colour that they bear.

3. Confusion of Blue and Black.-This variety of colourblindness is much less common than either of the preceding two. It is met with in cases in which there is a large neutral band extending to the violet. All blues will appear gray, and a very dark blue will not be distinguished

from black. In addition to this it must be remembered that a dark pure blue, the colour of which is obvious enough by daylight, is not easily distinguished from black by gaslight by the normal-sighted.

4. Confusion of Green and Black.-Green is not easily distinguished from black by those two-units who have a neutral band in the blue-green. The green which they confuse with black will be a green, corresponding to the portion of the spectrum occupied by the neutral band. The green will really appear as a very dark gray, which can be distinguished from black by comparison, but under ordinary circumstances would be called black without hesitation. The larger the neutral band the greater the variety of greens which will be indistinguishable from black. In all these cases the greens will be dark for the reason mentioned, when speaking of blue.

5. Confusion of other Colours with Black.-In the twounit any two colours taken from the two-units will make gray. It is obvious, therefore, that a dark purple which is confused with blue-green will also be confused with black.

II. Confusion of one Colour with another. The cases that are included under this head may be produced by shortening of the spectrum or diminution in psycho-physical perception. The confusion of colours due to psychophysical perception is caused by colours which appear perfectly distinct to the normal-sighted, being included in a single unit of the colour-blind. Confusion of colours caused by a shortened spectrum is due to the fact that coloured substances, as seen by the normal-sighted, reflect other rays than those of the colour which they appear. In these cases the colour which, to the normal-sighted, appears the lighter of the two will be found to reflect rays occupying the shortened portion of the spectrum. If these