of cardboard painted with alternate white and black sectors. like that shown in Fig. 26, be set in rotation while exposed to full daylight, colours will be seen after a few trials. It will be found that a certain rate of rotation communicates to the disk a green hue, a somewhat more rapid rate causing it to assume a rose colour. According to Helmholtz, these effects are most easily attained by using a disk painted with a black spiral, like that in Fig. 27. These phenomena may be advantageously studied by a method which was used by the author several years ago. A blackened disk with four open sectors seven degrees in width was set in revolution by clockwork, and a clouded sky viewed through it. With a rate of nine revolutions per second, the whole sky often appeared of a deep crimson hue, except a small spot in the centre of the field of view, which was pretty constantly yellow. Upon increasing the velocity to eleven and a half revolutions per second, the central spot enlarged somewhat, and became coloured bluish green, with a narrow, faint, blue border, indicated by the dotted line; the rest of the sky appeared purple, or reddish purple. (See Fig. 28.) With the exception of fluctuations in the outline of the spot, this appearance remained tolerably constant as long as the rate of revolution was steadily maintained. When the velocity * of the disk was increased, the bluish-green spot expanded into an irregularly shaped blue-green ring, which with a rate of fifteen turns per second mostly filled the whole field of view. (See Fig. 29.) With higher rates all these appearances vanished, and the sky was seen as with the naked eye. More than one elaborate attempt has been made to found on phenomena of this class a theory of the production of colour, though it may easily be shown that in all such cases the disk really transmits not coloured but white light, and that the effects produced are due to an abnormal state of the retina caused by alternate exposure to light and darkness. A current of electricity is also capable of stimulating the optic nerve in such a way that brilliant colours are perceived, although the experiment is made in perfect darkness. If the current of a strong voltaic battery be caused to enter the forehead, and travel hence to the hand, according to Ritter, a bright-green or bright-blue colour is perceived, the hue depending on whether the positive current enters the hand or forehead. Helmholtz, in repeating this operation, was conscious simply of a wild rush of colours without order. The experiment is, however, interesting to us, as proving the possibility of the production of the sensation of colour without the presence or action of light. Recently a substance has been discovered which, when swallowed, causes white objects to appear coloured greenish yellow, and coloured objects to assume new hues. Persons under the influence of santonin cannot sec the violet end of the spectrum; and this fact, with others, proves that they have become temporarily colour-blind to violet. An observation of Tait's, and others by the author, have shown that a shock of the nervous system may produce momentarily colour-blindness to green light. White objects then appear of a purplish red, and green objects of a much * "American Journal of Science and Arts," September, 1860. duller green hue than ordinarily.* These effects are evanescent, though quite interesting, as we shall see presently, from a theoretical point of view. Investigations during the present century have shown that many persons are born with a deficient perception of colour. In some the defect is slight and hardly noticeable, while in others it is so serious as to lead to quite wonderful blunders. This imperfection of vision is often inherited from a parent, and may be shared by several members of the same family. It is remarkable that women are comparatively free from it, even when belonging to families of which the male members are thus affected. The occupations of women, their attention to dress and to various kinds of handiwork where colour enters in as an important element, seem to have brought their sense for colour to a higher degree of perfection than is the case with men, who ordinarily neglect cultivation in this direction. Out of forty-one young men in a gymnasium, Seebeck found five who were colour-blind; but during his whole investigation he was able to learn of only a single case where a woman was to some extent similarly affected. It not unfrequently happens that persons with this defect remain for years unconscious of it. This was the case with some of the young men investigated by Seebeck; and in one remarkable instance a bystander, in attempting to help a colour-blind person who was under investigation, showed that he was himself colourblind, but belonged to another class! The commonest case is a deficient perception of red. Such persons make no distinction between rose-red and bluish-green. They see in the spectrum only two colours, which they call yellow and blue. Under the name yellow they include the red, orange, yellow, and green spaces: the blue and violet they name, with some correctness, blue. In the middle of the spectrum * "American Journal of Science and Arts," January, 1877. A similar observation by Charles Pierce was communicated to the author while this work was going through the press. there is for them a neutral or grey zone, which has no colour; this, according to Preyer, is situated near the line F. For the normal eye it is greenish-blue; for them, white. The extreme red of the spectrum, when it is faint, they fail to distinguish; the rest of the red space appears to them of a saturated but not luminous green; the yellow space has for them a colour which we should call bright green; and finally, they see blue in the normal manner. Maxwell found that by the aid of his disks, using only two colours, along with white and black, he was able, by suitable variations in their proportions, to match for them any colour which presented itself; while the normal eye requires at least three such coloured disks, besides white and black. His experiments led to the result that persons of this class perceive two of the three fundamental colours which are seen by the normal eye. Helmholtz also arrived at the same result. It is possible to render the normal eye to some extent colourblind to red in the manner followed by Seebeck in 1837, and afterward by Maria Bokowa. These observers wore for several hours spectacles provided with ruby-red glasses; and this prolonged action of the red light on the eye finally, to a considerable extent, tired out the nerve fibrils destined for the reception of red, so that on removing the glasses they saw in the spectrum only two colours. The second observer called them yellow and blue. Furthermore, the extreme red end of the spectrum was not visible to her, just as is the case with those who are actually blind to red; all red objects appeared to her yellow, and dark red was not distinguishable from dark green or brown. Dalton, the celebrated English chemist, suffered from this defect of vision, and was the first to give an accurate description of it; hence this affection is sometimes named after him, Daltonism. It is very remarkable that, according to the observations of Schelske and Helmholtz, even in the normal eye there are portions which are naturally colourblind to red, and when this zone of the eye is used the same mistakes in matching colours are made. Such experiments are somewhat difficult to make without considerable practice, as it is necessary that the colored objects should be viewed, not directly, but by the eye turned aside somewhat. There is a simple means by which persons who are colourblind to red can to some extent help themselves, and prevent the occurrence of coarse chromatic blunders, such as confusing red with green. Green glass does not transmit red light; hence, on viewing green and red objects through a plate of this glass it will be found, even by persons who are colour-blind, that the red objects appear much more darkened than those which are green. On the other hand, a red glass will cause green objects to appear darker, but will not affect the luminosity of those having a tint similar to itself. The exact tints of the glasses are important, and they should of course be selected with the aid of a normal eye. The kind of colour-blindness just described is rather common, and it has been estimated that in England about one person in eighteen is more or less afflicted with it. We pass on now to the consideration of a class of cases that is more rare. Persons belonging to this second class see only two colours in the spectrum, which they call red and blue. They set the greatest luminosity in the spectrum in the yellow space, as is done by the normal eye; and they easily distinguish between red and violet, but confuse green with yellow and blue with red. In two cases examined by Preyer, yellow appeared to them as a bright red ; this same observer also found that in the spectrum, near the line b, the two colours into which they divided the spectrum were separated by a small neutral zone, which was for them identical with grey. A sufficient number of observations have not been accumulated to furnish means of ascertaining with certainty the exact nature of the difficulty under which they labour, though it is probable that they are colour-blind to green light. There are also observations on record of cases of temporary colour-blindness of a third kind, where the violet |