ISO Experimental Philosophy. [Lecture 10. 



is done by placing them over a gentle fire, and 

 leaving them thus to evaporate in a close room 

 without a chimney: if a person should soon after 

 enter with a candle, he will find the whole room 

 filled with innoxious flames. The parts have 

 been too minutely separated, and the fluid, per- 

 haps, has not force enough to send forth its 

 burning rays with sufficient effect." 



It is not, however, my intention in these lectures 

 to involve you in the intricacies of theory, or to 

 pursue speculative inquiries at the expense of 

 useful facts. It will be more profitable to detail 

 and explain the properties of light than to waste 

 our time in conjectures on its essence. The most 

 remarkable properties of light, then, are, first, 

 itsveloci/y; secondly, its rarity; thirdly, its force 

 or momentum; fourthly, the property of being 

 always detached in straight lines ; fifthly, refrac- 

 tion; and, sixthly, the reflection of light. 



I. The velocity of light is such as may well 

 astonish the inexperienced student, when he is 

 told that in the very short space of a moment, or 

 a second of time, a ray of light travels the im- 

 mense extent of one hundred and seventy thousand 

 miles. The manner in wliich the velocity of 

 light is calculated is not less ingenious than the 

 discovery is surprising. It was by observing the 

 eclipses of Jupiter's satellites, and it will be 

 amusing to you to observe the process by which 

 the calculation is accomplished. When the earth, 

 in going its annual revolution round the sun, is 



