430 SPECIAL PHYSIOLOGY. 



Light. 



Those bodies, such as the sun and fixed stars, from which light 

 directly emanates, are called self-luminous bodies. The sun is the 

 chief source of light, but there are also terrestrial sources of light, 

 such as combustion and other chemical combinations, friction, and 

 electricity. Non-luminous bodies are only rendered visible by the 

 light which falls upon them from luminous bodies. 



According to the Newtonian theory of emanation, light is of a cor- 

 puscular nature, a luminous body constantly emitting material particles 

 in all directions. The undulatory theory of light, first suggested by 

 Huyghens, and now generally adopted, supposes, that space is filled 

 with an ether, which, when thrown into exceedingly rapid undulations 

 gives rise in the eye to the sensation of light, as the vibrations of ma- 

 terial particles, communicated to the ear, produce the sensation of 

 sound. In accordance with the doctrine of the correlation of forces, 

 the undulations of light are supposed to result from one of the modes 

 of action of the general force of nature, and therefore to be able to 

 originate in mechanical, chemical, thermal, or electrical modes of 

 action. 



The undulations of the luminiferous ether being supposed to be prop- 

 agated from a luminous point in all directions, the term ray, a con- 

 ventional but convenient expression, is applied to any imaginary line 

 drawn from such a point across the waves, that is, perpendicular to 

 their expanding and advancing fronts. The undulations are therefore 

 propagated in the direction of such lines or rays ; but the motions of 

 displacement of the ether which produce those waves, are transverse 

 to the direction of the rays. 



The so-called rays of light move in straight lines; their rate of mo- 

 tion, in space, formerly estimated at 192,500 miles, is now said to be 

 about 186,300 miles per second; their velocity is retarded in dense 

 media. Light radiates equally in all directions, and, by spreading, 

 its luminous power diminishes as the square of the distance through 

 which it passes. When it falls upon any surface, it may be either re- 

 flected or absorbed. The angle of reflection is equal to the angle of 

 incidence. The reflection is said to be regular, when the reflected 

 light from an opaque body with a polished surface, produces images 

 of objects placed in front of it ; by multiplying such surfaces, the re- 

 flected images are broken ; and, if the surface be rough, no image is 

 formed, the light is scattered, and its reflection is irregular. It is by 

 the reflection of the'light which impinges upon non-luminous, opaque, 

 or transparent objects, from luminous bodies, that the former are ren- 

 dered visible. Such objects, when they reflect light, completely or 

 almost completely, present a white or whitish appearance: but, if there 

 is complete or almost complete absorption of light, they assume a black 

 or blackish appearance. The interception of a portion of the rays of 

 light by opaque bodies, is the cause of shadows. When light falls on 

 a translucent body, it is partly reflected, partly transmitted, and partly 

 absorbed ; when it falls directly on a transparent body, such as air, 



