The soluble portion of the lens protein has been the subject of considerable interest and study since the time of Moerner, 1 who showed that it was composed almost entirely of the two crystallines which represented in great part the sulphur content. The nitroprusside reaction, which was described by Arnold 2 in 1911, showed that the normal lens when treated with a 5 per cent solution of sodium nitroprusside and a drop of ammonia will turn a decided red, resembling the color of permanganate. In studying senile cataract, Reiss 3 and Jess 4 found that whereas this reaction was positive with the normal lens, it was negative in those parts of the lens that had become cataractous. In mature senile cataract, the red color which was produced ordinarily was entirely absent. Since the positive reaction was said to be caused by the amino-acid cysteine, it was concluded