Effect of Bodies on Light and Heat. 387 



locity in going through these transparent bodies, a reduction made by a 

 reduction of wave-length but not of rapidity of vibration. Refraction is 

 a consequence of the passage of the vibratory movement from a medium 

 of one density to that of another. In going from the dense to the rare, 

 unless it strikes perpendicularly to the surface, the beam of light is bent 

 or refracted from the normal, or plane perpendicular to the refracting 

 surface, while in going from the rare to the dense, the refraction is to- 

 ward the normal. 



Fia. 160. Glass Prism showing refraction of 

 rays in passing through. 



The ray from the object, o, upon entering the 

 prism is Dent and its track through is deflected 

 toward the normal n n. On leaving, it is again 

 deflected, this time away from the normal, so 

 that the eye sees o in the direction of s. 



A study of fig. 160 and a comparison 



FIG. 160. with fig. 158 will show that the same 



principle governs refraction in both light and sound. The waves are 

 shortened by going through the denser medium, and the greater the dis- 

 tance through, the more the beam will be shortened. The lower part of a 

 beam, from o to e, fig. 160, will have a greater length in the glass than 

 the upper part, and so the lower part will be made shorter, which will 

 make the beam bend. If another prism were inverted and placed with 

 its base up against that of the first, the object o would be seen at e by 

 a ray passing through that prism also, but it would be bent in the op- 

 posite direction, and the two acting together thus, plainly indicate the 

 principle of the lens. 



The refraction of light in passing through a transparent gas is in- 

 creased if the gas be condensed in bulk under pressure. In general, 

 the greater the difference in density between two mediums through 

 which the light passes, the greater the refraction in the passage from 

 one to the other. The vibrations of radiant energy ( heat and light ) 

 are at fight angles, or normal, to the direction in which it moves. A 

 fiddle string in vibrating may fly in any direction from its silent posi- 

 tion ; viz. , to the right or left, or up or down, or at any angle interme- 

 diate of these. These various directions are called azimuths. Vibra- 

 tions of ether must be conceived in like manner to take place alternately 

 or under varying conditions in any and all azimuths. 



CHAPTER XLI. 



EFFECT OF BODIES ON LIGHT AND HEAT. 



After being assured that light is not a substance, but merely the mo- 

 tion of a body entirely impalpable and invisible, it is somewhat diffi- 

 cult to realize that such motion could in any w:iy affect or be affected by 

 ponderable bodies. But in fact, radiant energy, which term comprises 



