202 THE POPULAR SCIENCE MONTHLY. 



enough, and in photographing the sun itself 6o fl 00 of a second is 

 sufficient. 



We owe to Wheatstone the conception that the idea of solidity is 

 derived from the combination of two pictures of the same object in 

 slightly different perspective. This he proved in 1833 by drawing two 

 outlines of some geometrical figure or other simple object, as they 

 would appear to either eye respectively, and then placing them so that 

 they might be seen, one by each eye. The " stereoscope," thus pro- 

 duced, has been greatly popularized by photography. 



For two thousand years the art of lighting had made little if any 

 progress. Until the close of the last century, for instance, our light- 

 houses contained mere fires of wood or coal, though the construction 

 had vastly improved. The Eddystone Lighthouse, for instance, was 

 built by Smeaton in 1759 ; but for forty years its light consisted in 

 a row of tallow candles stuck in a hoop. The Argand lamp was the 

 first great improvement, followed by gas, and in 1863 by the electric 

 light. 



Just as light was long supposed to be due to the emission of 

 material particles, so heat was regarded as a material, though ethereal, 

 substance, which was added to bodies when their temperature was 

 raised. 



Davy's celebrated experiment of melting two pieces of ice by rub- 

 bing them against one another in the exhausted receiver of an air- 

 pump, had convinced him that the cause of heat was the motion of the 

 invisible particles of bodies, as had been long before suggested by 

 Newton, Boyle, and Hooke. Rum ford and Young also advocated 

 the same view. Nevertheless, the general opinion, even until the 

 middle of the present century, was that heat was due to the presence 

 of a subtile fluid known as " caloric," a theory which is now entirely 

 abandoned. 



Melloni, by the use of the electric pile, vastly increased our knowl- 

 edge of the phenomena of radiant heat. His researches were confined 

 to the solid and liquid forms of matter. Tyndall studied the gases in 

 this respect, showing that differences greater than those established by 

 Melloni existed between gases and vapors, both as regards the absorp- 

 tion and radiation of heat. He proved, moreover, that the aqueous 

 vapor of our atmosphere, by checking terrestrial radiation, augments 

 the earth's temperature, and he considers that the existence of tropical 

 vegetation the remains of which now constitute our coal-beds may 

 have been due to the heat retained by the vapors which at that period 

 were diffused in the earth's atmosphere. Indeed, but for the vapor in 

 our atmosphere, a single night would suffice to destroy the whole vege- 

 tation of the temperate regions. 



Inspired by a contemplation of Graham Bell's ingenious experi- 

 ments with intermittent beams on solid bodies, Tyndall took a new 

 and original departure ; and regarding the sounds as due to changes 



