1. _Dichromic vision._--The cases which come under this head form the class of the ordinary red-green blind. It is under this head that nearly every one of the recorded cases may be classed. Vision, as far as colour is concerned, is dichromic, the neutral point being situated in the green of the normal-sighted at about λ 500. All the colours on the red side tend to be confused with each other; therefore red, orange, yellow and half of the green are seen as one colour, the remainder of the green, blue and violet as the other. The luminosity curve in uncomplicated cases is similar to the normal. There may be shortening of the spectrum at either the red or the violet end of a varying degree. All degrees of shortening of the red end of the spectrum may be found. Dichromics with normal luminosity curves are those which were formerly designated green-blind; but this designation is not in accordance with the facts, because there is no defect of light-perception in the green, and the so-called diagnostic mistakes, as, for instance, putting a bright green with a dark red, are not made. Cases of so-called red-blind are dichromics with shortening of the red end of the spectrum. I have shown that the defective perception of the red end of the spectrum will not account for the dichromic vision which is found in these cases. We may also meet with shortening of the spectrum with otherwise normal colour-perception. We also meet with dichromic cases forming a series from almost total colour-blindness to those bordering on the trichromic. Any theory must account for the fact that there are varying degrees of colour-blindness in dichromic vision, and why there is a large neutral band corresponding to the colours of the centre of the spectrum in some cases, and in others the neutral band is so small that the dichromic cannot mark it out. The two colours seen by the dichromic are red and violet, though where no distinction is seen between yellow and red, and blue and violet, the brighter colour will often be selected; that is why so many dichromics say that they see yellow and blue in the spectrum. Those who have had practical experience of colour-blindness will know, however, that many dichromics make no mistakes with the red test. The following will give the normal-sighted the best idea of colour-blindness, and it is how the dichromic see the spectrum, and explains why they are able to distinguish between colours. Let him regard the dichromic as a man who has two colours--red and violet and white. The purest red is at the red end of the spectrum; this becomes less and less saturated as the violet is approached until the neutral or white point is reached; then violet comes into the white, and this increases in saturation to the termination of the violet. The ordinary dichromic therefore sees green as a much whiter and less saturated colour than red.