435 '^tECTURE XXXVII. 



cious in exciting in a diamond the appearance of another kind of light, which 

 it was naturally most disposed to exhibit. The application of heat to 

 solar phosphori in general expedites the extrication of the light which they 

 have borrowed, and hastens its exhaustion; it also produces, in many sub- 

 stances, which are not remarkable for their power of imbibing light, a tem- 

 porary scintillation, or flashing, at a heat much below ignition: the most re- 

 markable of these are fluor spar in _po\\^der, and some other crystallized 

 substances. It appears that luminous bodies in general emit light equally in 

 every direction, not from each point of any of their surfaces, as some have 

 supposed, but from the whole surface taken together, so that the surface, 

 when viewed obliquely, appears neither more nor less bright than when viewed 

 directly. 



However light of any kind may have at first originated, there is reason 

 to believe that the velocity with which it passes through a given medium 

 is always the same. It has been ascertained by the astronomical ob- 

 servations of Roemer and of Bradley, that each ray of light, emitted by the 

 sun, arrives at the earth in eight minutes and one eighth, when the 

 earth is at its mean distance of about 95 millions of miles. Roemer deduced 

 this velocity from observations on the eclipses of the satellites of Jupiter, 

 and Bradley confirmed it by his discovery of the cause of the apparent aber- 

 ration of the fixed stars. 



This aberration is produced by the eflfect of the revolution of the earth in its 

 orbit, combined with that of the progressive motion of light. Since light 

 proceeds always in right lines, when its motion is perfectly undisturbed, if 

 a fine tube were placed so as to receive a ray of light, passing exactly 

 through its axis when at rest, and then, remaining in the same direction, 

 were moved transversely with great velocity, it is evident that the side 

 of the tube would strike against the ray of light in its passage, and that in 

 order to retain it in the axis, the tube must be inclined, in the same manned" 

 as if the light, instead of coming in its actual direction, had also a transverse 

 motion in a contrary direction to that of the tube. The axis of a telescope, 

 or even of the eye, may be considered as reseuibling such a tube, the passage 

 of the light through tlie refracting substances not altering the necessary in- 

 clination of the axis. In various parts of the earth's orbit, the aberration 



