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.

O. N. R.

EFFECTS PRODUCED ON COLOUR BY A CHANGE IN LUMINOSITY AND BY

MODERN CHROMATICS.

CHAPTER I.

THE REFLECTION AND TRANSMISSION OF LIGHT.

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. the great majority of instances, however, the stimulation of the optic nerve is brought about, directly or indirectly, by the

In

aid of light; and in the present work it is principally with vision produced in this normal manner that we have to deal.

Back in the rear portion of the eye there is spread out a delicate, highly complicated tissue, consisting of a wonderfully fine network woven of minute blood-vessels and nerves, and interspersed with vast numbers of tiny atoms, which under the microscope look like little rods and cones. This is the retina; its marvellous tissue is in some mysterious manner capable of being acted on by light, and it is from its substance that those nerve-signals are transmitted to the brain which awake in us the sensation of vision. For the sake of brevity, the interior globular surface of the retina is ordinarily called the seat of vision. An eye provided only with a retina would still have the capacity for a certain kind of vision; if plunged in a beam of red or green light, for example, these colour-sensations would be excited, and some idea might be formed of the intensity or purity of the original hues. Some of the lower animals seem to be endowed only with this rudimentary form of vision; thus it has lately been ascertained by Bert that minute crustaceans are sensitive to the same colours of the spectrum which affect the eye of man, and, as is the case with him, the maximum effect is produced by the yellow rays. With an eye constructed in this simple manner it would, however, be impossible to distinguish the forms of external objects, and usually not even their colours. We have, therefore, a set of lenses placed in front of the retina, and so contrived as to cast upon it very delicate and perfect pictures of objects toward which the eye is directed; these pictures are coloured and shaded, so as exactly to match the objects from which they came, and it is by their action on the retina that we see. These retinal pictures are, as it were, mosaics, made up of an infinite number of points of light; they vanish with the objects producing themthough, as we shall see, their effect lasts a little while after they themselves have disappeared.