IT was not my intention to write a preface to this book, is I have usually found such compositions neither instructive nor amusing. On presenting the manuscript to my publishers, however, it was suggested that, al though prefaces are of no particular use to readers, yet from a certain point of view they are not without value. I accordingly beg leave to state that my object in this work has been to present, in a clear, logical, and if possi ble attractive form, the fundamental facts connected with our perception of colour, so far as they are at present known, or concern the general or artistic reader. For the explanation of these facts, the theory of Thomas Young, as modified and set forth by Helmholtz and Maxwell, has been consistently adhered to. The whole class of musical theories, as well as that of Field, have been discarded, for reasons that are set forth in the text.