462 THE POPULAR SCIENCE MONTHLY. 



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 conduc- 

 tors ; their irritation, when it does take place, necessarily produces im- 

 pressions which come under the head of qualitative, for the eye, there- 

 fore, of the quality of luminous sensations ; those impressions do not, 

 however, stand in any closer relation to the adequate sensory irrita- 

 tion, and may be, as far as we are concerned, quite devoid of sensitive- 

 ness. 



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 ve- 

 locity 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 irri- 

 tation 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 them- 

 selves 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 ob- 

 jects, 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 



