Page:Popular Science Monthly Volume 1.djvu/476

462 the irritable nervous matter, and at once takes its place among the mechanical excitations mentioned above when speaking of subjective sight.

The cord of the optic nerve is insensible to the undulations of the ether; the peripheral expansion of the retina is alone susceptible of irritation by light. This peculiarity has to do with the arrangement called the terminal apparatus, with which it is now proved every nerve is furnished. The nervous cords themselves are preeminently conductors; their irritation, when it does take place, necessarily produces impressions which come under the head of qualitative, for the eye, therefore, of the quality of luminous sensations; those impressions do not, however, stand in any closer relation to the adequate sensory irritation, and may be, as far as we are concerned, quite devoid of sensitiveness.

According to physics, light is the same species of motion in the ether as warmth; only, in order to affect the retina, the undulations must take place within certain limits of rapidity. Relatively they possess the greatest velocity in the violet-colored part of the spectrum, and the least in the red. In the same proportion as the velocity of the undulations diminishes, does light become invisible, and only dark rays of warmth are emitted, while on its being more highly heated, from the increased rapidity of the undulations, it reaches the glowing-point, that is, it emits rays of red light.

From this we see that the idea of light depends essentially on the organization of the retina. Were it different from what it is, did it possess any susceptibility for ethereal vibrations of a less degree of velocity than those at the red end of the spectrum, then we should call that light which we now term a dark warmth.

In certain cases of natural color-blindness, the susceptibility of irritation in the retina is quite undeveloped for the extremest red of the spectrum.

Now, while the light coming from external objects irritates the retina variously according to its color and power, the impressions made by luminous objects are also very various, and herein lies the first link of connection with the outer world.

It is only the visual organs of the lower animals which lose themselves in such a general and vague relation to the surrounding ocean of light and color. The organ which now occupies our attention has a far higher design to serve, viz., to awaken a perception of separate objects, and of their peculiar forms and colors. Were the retina as you see it in Fig. 1, a surface curving outward, then such a design could not be fulfilled; for every part would receive light from all the points of the outside world. In order to fulfil this condition, every individual point of the retina must enter into a separate and individual relation with the light proceeding from a point beyond it; nor till this takes