different parts of the same cone vibrate with different degrees of rapidity, and therefore respond to different colors, and that the conical form is adapted for this purpose. In order to gain clearer conception, we may imagine each cone to be made up of a number of buttons of graduated sizes joined together. These buttons, on account of their different sizes, would vibrate with different degrees of rapidity, and therefore co-vibrate with different colors. White light, he supposes, vibrates the whole series; red light, the thicker, and violet, the thinner, portion of the series; or, taking Hering's view of the primary colors, we may imagine that red and green rays affect one portion, and yellow and blue rays another portion, of the same cone.

The subject of the mechanism of color perception, however, is yet in the region of speculation, though probably of profitable speculation. To pursue it any further would be unsuited to the character of this treatise.

Daltonism, or Color-Blindness.-Many persons lack a nice discrimination of shades of color. Such persons. see colors perfectly well, but, from want of attention or culture, have not learned to nicely discriminate and name them. This must not be confounded with colorblindness. The color-blind do not see some colors as colors at all. The defect is not one of culture, but of sensation. We can best explain it by comparing the eye and ear.

The limits of the perception of sound-vibrations are very wide, viz., more than eleven octaves. The limits. of perception of light-vibrations are far more restricted, viz., only a little more than one octave. Now in many ears the extreme limits are not perceived; but this is not considered a defect, because there is no special use for

the extremest range. So in the eye. Even the narrow limits of the normal eye are sometimes not reached; but in this case the usefulness of the whole range makes it a serious defect. This is color-blindness. In the ear the vibrations most commonly unperceived are at the upper end of the scale. In the eye it is usually the lower end of the scale which is defective, viz., red, or red and green. The color-blind see yellow and blue, but not red and green.

This defect was first brought to scientific notice by the celebrated chemist Dalton, and after him has often been called Daltonism. The peculiarities of Dalton's vision were carefully investigated by Sir John Herschel, and the first scientific explanation was given by him. Adopting the view of Young of three primary colors, Herschel regarded normal vision as trichromic, but the vision of Dalton as dichromic, the red being wanting. This view certainly explained the most striking phenomena of color-blindness, but it does not explain the fact that green is wanting as well as red. As shown by Pole* (who is himself color-blind), the phenomena are far more perfectly explained on Hering's view of the primary colors; and conversely, the phenomena of colorblindness are a powerful argument in favor of Hering's view. Of the two pairs of complementary colors of Hering, one pair, viz., the red-green, is wanting in the color-blind, while the other pair, yellow-blue, is perceived as in normal vision. The colors and shades, therefore, which are perceived by the color-blind are: 1, black and white, and all intermediate shades of gray; 2, yellow in all its shades; and, 3, blue in all its shades. A pure red seems to them a dark gray; but if mixed with yellow, as

* "Nature," 20, pp. 477, 611, 637 (1879); "Contemporary Revicw," May, 1880.

are most reds, it appears yellow mixed with gray, or a kind of brown; or if mixed with blue (purple), it appears as blue mixed with gray, or slate-blue. A pure green appears simple gray; a yellow-green, yellow mixed with gray-i. e., brown; and a blue-green, slate blue.

The cause of this defect of vision is, of course, a defect of retinal structure. If we admit that the rods and cones are the responsive elements, and that different kinds of rods or cones respond to different primary colors, then in the retina of the color-blind the rods or cones responding to red and green are wanting; or, by Hall's theory, the cones are so shaped that they respond to only one complementary pair, viz., to yellow and blue.

SECTION II-FUNCTION OF THE RETINA, AND EXPLANATION OF THE PHENOMENA OF MONOCULAR VISION.

There is a certain peculiarity in the general function of the retina, optic nerve, and associated brain apparatus, which must now be explained and clearly apprehended, in order to understand the phenomena of vision.

Law of Outward Projection of Retinal Impressions.An image is formed on the retinal screen. We have seen that the whole object of the complex arrangement of lenses placed in front of the retina is the formation of images. But we do not see the retinal images. We do not see anything in the eye, but something outside in space. It would seem, then, that the retinal image impresses the retina in a definite way; this impression is then conveyed by the optic nerve to the brain, and determines changes there, definite in proportion to the

distinctness of the retinal image; and then the brain or the mind refers or projects this impression outward into space as an external image, the sign and facsimile of an object which produces it. We shall see hereafter how important it is that we regard what we see as external images, the signs of objects which produce them, and these external images themselves as projections outward of retinal images.

This law of outward projection is so important that we will stop a moment to show that it is not a new law specially made for the sense of sight, but only a modification of a general law of sensation. After doing so, we will proceed to illustrate by many phenomena, so as to fix it well in the mind.

Comparison with Other Senses.-The general law of sensation is, that irritation or stimulation in any portion of the course of a sensory fiber is referred to its peripheral extremity. Thus, if the sciatic nerve be laid bare in the upper thigh, and then pinched, the pain is felt, not at the part injured, but at the termination of the nerve in the feet and toes. If the ulnar nerve be pinched in the hollow on the inner side of the point of the elbow, pain is felt in the little and ring fingers, where this nerve is distributed. In amputated legs, as is well known, the sense of the presence of a foot remains, and often severe neuralgic pains are felt in the feet and toes. The pain, which in this case is caused by a diseased condition of the nerves at the point of amputation, is referred to the place where the diseased fibers were originally distributed. In nerves of common sensation, therefore, injury or disease, or stimulation of any kind in any part, is referred to the peripheral extremity of the nerve-fibers. Now the peculiarity of the optic nerve is, that it refers impres

sions not to its peripheral extremity only, but beyond

into space.

But when we find great differences in the functions of tissues, such as occur in this case, we can generally find the steps which fill up the gap. A thoughtful comparison of the phenomena of the different senses will, we believe, reveal these steps. We repeat here what has already been said in a general way on page 13. Commencing with the lowest of the specialized senses, the gustative, an impression on the nerves of taste is referred, as in the case of common sensory fibers, to their peripheral extremity: the sensation is on the tongue. In the case of the olfactive, we have a sensation still at the peripheral extremity, i. e., in the nose, but also a reference to an external body at a distance as its cause. Here the objective cause and the subjective sensation are separated, and both distinct in the mind. In the case of the auditive nerve, the sensation is no longer perceived, or at least is very imperfectly perceived, in the ear, but is nearly wholly objective, i. e., referred to the distant sounding body. Finally, in the case of the optic nerve, the impression is so wholly projected outward that the very reminis cence of its subjectivity is entirely lost. We are perfectly unconscious of any sensation in the eye at all.

Illustrations of this Property.-We will now try to make this property clear by many illustrative experi

ments.

Experiment 1.-If the retina or the optic nerve in any portion of its course were irritated in any way, by pinching, by scratching, or by electricity, we should certainly not feel any pain at all, but see a flash of light. But where? Not at the peripheral extremity only, not in the eye, but beyond in the field of view. Of course,