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It was not my intention to write a preface to this book, as I have usually found such compositions neither instructive nor amusing. On presenting the manuscript to my publishers, however, it was suggested that, although 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 possible 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.

Turning now from the more purely scientific to the æsthetic side of the subject, I will add that it has been my endeavour also, to present in a simple and comprehensible manner the underlying facts upon which the artistic use of colour necessarily depends. The possession of these facts will not enable people to become artists ; but it may to some extent prevent ordinary persons, critics, and even painters, from talking and writing about colour in a loose, inaccurate, and not always rational manner. More than this is true: a real knowledge of elementary facts often serves to warn students of the presence of difficulties that are almost insurmountable, or, when they are already in trouble, points out to them its probable nature; in short, a certain amount of rudimentary information tends to save useless labour. Those persons, therefore, who are really interested in this subject are urged to repeat for themselves the various experiments indicated in the text.

In the execution of this work it was soon found that many important gaps remained to be filled, and much time has been consumed in original researches and experiments. The results have been briefly indicated in the text; the exact means employed in obtaining them will be given hereafter in one of the scientific journals.

To the above I may perhaps be allowed to add, that during the last twenty years I have enjoyed the great privilege of familiar intercourse with artists, and during that period have devoted a good deal of leisure time to the practical study of drawing and painting.

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As long ago as 1795 it occurred to a German physicist to subject the optic nerve of the living eye to the influence of the newly discovered voltaic current. The result obtained was curious : the operation did not cause pain, as might have been expected, but a bright flash of light seemed to pass before the eye. This remarkable experiment has since that time been repeated in a great variety of ways, and with the help of the more efficient electric batteries of modern times; and not only has the original result of Pfaff been obtained, but bright red, green, or violet, and other hues have been noticed by a number of distinguished physicists. If, instead of using the electrical current, mechanical force be employed, that is, if pressure be exerted on the living eye, the optic nerve is again stimulated, and a series of brilliant, changing, fantastic figures seem to pass before the experimenter. All these appearances are distinctly visible in a perfectly dark room, and prove that the sense of vision can be excited without the presence of light, the essential point being merely the stimulation of the optic nerve. In the great majority of instances, however, the stimulation of the optic nerve is brought about, directly or indirectly, by the

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