Light. 141 



this hypothetical computation, expended from the 

 sun's surface, or six hundred and seventy pounds 

 in six thousand years, which would have shortened 

 his diameter about ten feet, if it were formed 

 of matter of the density of water only. From 

 all this you will conclude that I have adopted 

 the common theory, that the sun is the great 

 source of light ; and if his diameter is rightly 

 calculated (of which there can be no doubt) at 

 eight hundred and seventy-eight thousand eight 

 hundred and eight miles, we see there is no 

 ground for any apprehensions that the sun will 

 speedily be exhausted by the waste or consump 

 tion of light. The matter will not be widely dif 

 ferent, if we imagine, as is now generally believed, 

 that the particles of light are emitted from a 

 luminous atmosphere which surrounds the body 

 of the sun. 



IV. Another principle to which I proposed to 

 call your attention is, that light always moves in 

 straight lines. This is evident from an experi- 

 ment which any person may easily make, viz. 

 that of looking through a bent tube, when no 

 light whatever will be apparent. As a further 

 proof it is only necessary to mention, that when 

 light is intercepted by any intervening body, the 

 shadow is bounded by straight lines. 



It is generally supposed, according to this 

 principle, that those bodies only are transparent 

 whose pores are such as to permit the rays of 

 light to pervade them in a rectilinear direction ; 



