appears to me that of a dull, opaque, blackish blue upon a white ground. Dilute black ink upon white paper gives a colour much resembling that of a florid complexion. It has no resemblance to the colour of blood." We have a detailed account of the case of a young Swiss, who did not perceive any great difference between the colour of the leaf and that of the ripe fruit of the cherry, and who confounded the colour of a sea-green paper with the scarlet of a riband placed close to it. The flower of the rose seemed to him greenish blue, and the ash gray colour of quick-lime light green. On a very careful comparison of polarized light by the same individual, the blue, white, and yellow were seen correctly, but the purple, lilac, and brown were confounded with red and blue. There was in this case a remarkable difference noticed according to the nature and quantity of light employed; and as the lad seemed a remarkably favourable example of the defect, the following curious experiment was tried. A human head was painted, and shown to the colour-blind person, the hair and eyebrows being white, the flesh brownish, the lips and cheeks green. When asked what he thought of this head? the reply was, that it appeared natural, but that the hair was covered with a nearly white cap, and the carnation of the cheeks was that of a person heated by a long walk.
There is an interesting account in the Philosophical Transactions for 1859 (p. 325), which well illustrates the ideas entertained by persons in this condition with regard to their own state. The author, Mr. W. Pole, a well-known civil engineer, thus describes his case:—"I was about eight years old when the mistaking of a piece of red cloth for a green leaf betrayed the existence of some peculiarity in my ideas of colour; and as I grew older, continued errors of a similar kind led my friends to suspect that my eyesight was defective; but I myself could not comprehend this, insisting that I saw colours clearly enough, and only mistook their names.
"I was articled to a civil engineer, and had to go through many years' practice in making drawings of the kind connected with this profession. These are frequently coloured, and I recollect often being obliged to ask in copying a drawing what colours I ought to use; but these difficulties left no permanent impression, and up to a mature age I had no suspicion that my vision was different from that of other people. I frequently made mistakes, and noticed many circumstances in regard to colours, which temporarily perplexed me. I recollect, in particular, having wondered why the beautiful rose light of sunset on the Alps, which threw my friends into raptures, seemed all a delusion to me. I still, however, adhered to my first opinion, that I was only at fault in regard to the names of colours, and not as to the ideas of them; and this opinion was strengthened by observing that the persons who were attempting to point out my mistakes, often disputed among themselves as to what certain hues of colour ought to be called." Mr. Pole adds that he was nearly thirty years of age when a glaring blunder obliged him to investigate his case closely, and led to the conclusion that he was really colour-blind.
All colour-blind persons do not seem to make exactly the same mistakes,
