The brief time allotted for these lectures--an hour on three successive Saturday afternoons--restricted the discussion of some aspects of the question, and confined its treatment in the main to those features most readily explicable by the physicist, and to bringing into notice the latest results which had been obtained from physical experiments. How far the writer has succeeded in the task which he then outlined it is for the reader to determine. There was no intention in the first instance to publish these lectures. After their delivery, many persons expressed a desire that the information they contained should be rendered accessible to such as were interested in the theory of colour vision, and in deference to that desire the lecture-notes have been re-cast in book form. For the reader’s convenience the matter is now divided into chapters instead of into lectures, and a few additions have been made in the text to explain some of the experimental work to those who have not facilities for its repetition. The writer has to acknowledge several debts of gratitude. First, to Mr. E. Nettleship, for his kindness in looking over the proofs, and making valuable suggestions whilst the work was passing through the press; and also, as will be seen throughout its pages, for many of the interesting cases of defective colour perception which have been examined by the somewhat novel methods described. Next, the writer’s gratitude is due to Professor M. Foster for the permission he has given to use his admirable description of the Hering theory; and, lastly, to the Royal Society for the permission it accorded to use various diagrams which have served as illustrations to papers which have appeared in its “Philosophical Transactions” and “Proceedings.” COLOUR VISION. CHAPTER I. I must commence this course by saying that I feel the honour that has been done me in asking me to undertake it, connected as it is with the name of Tyndall, whose recent removal from our midst has been deplored by all lovers of science, and by none more than by those who have had the privilege of listening to him at this Institution. It is my duty to speak on some subject of physics, and the subject I have chosen is Colour Vision. I hope it will not be considered inappropriate, since it was Thomas Young, the physicist, whose connection with this Institution is well known, who first propounded a really philosophical theory of the subject. Interesting as it may be to trace how old theories have failed and new ones have started, I feel that for those who, like myself, have but little time at command in which to address you, the historical side of this question must of necessity be treated incompletely.