102 ELEMENTS OF SCIENCE 



It has been ascertained that some rays of heat which 

 have been reflected, or refracted, are not equally energetic 

 in all directions to which they may (by turning the reflect- 

 ing or refracting body) be directed ; but this phenomenon 

 will be best considered when we treat of light. 



LIGHT. There is great resemblance and accord 

 between many of the phenomena of light and those of 

 heat, and an analogous ignorance still exists about both. 

 Light, which reveals and makes known to us the world 

 in which we live, remains, like heat, itself unknown. 

 The problem as to its real nature is, however, one which 

 cannot here be entered on. 



Like heat, again, it was formerly supposed to consist 

 of minute material particles emitted (by luminous bodies) 

 in all directions, but afterwards it was regarded as the 

 effect of minute vibrations, or waves, of an elastic, 

 universally diffused ether. All that we need do here is 

 to welcome, as a working hypothesis, that representation 

 which best helps us to understand and anticipate the 

 phenomena which experience presents us with, keeping 

 an open mind about its truth, and being ready to lay it 

 down and make use of another hypothesis so soon as any 

 more serviceable one may be forthcoming. 



The close connection between light and heat is obvious 

 from the fact that we cannot have light (e.g., from fire 

 or candle) without having radiant heat also, and the 

 warmth to be felt by our body when exposed to the 

 brilliant light of the sun tells us the same thing. 



It is abundantly evident that whatever light may be in 

 itself, like heat, it diffuses itself in straight lines in all 

 directions from every visible object, and, of course, it also 

 comes in straight lines to the eye from every visible object. 



The light emitted from any point of any object is called 



