PHOTOGRAPHY. THE NATURE OF HEAT. 63 



of solidity is derived from the combination of two 

 pictures of the same object in slightly different perspec- 

 tive. 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 produced, has been greatly 

 popularised by photography. 



For 2,000 years the art of lighting had made little 

 if any progress. Until the close of the last century, 

 for instance, our lighthouses contained mere fires of 

 wood or coal, though the construction had vastly im- 

 proved. The Eddystone lighthouse, for instance, was 

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

 consisted of 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 rubbing them against one another in the ex- 

 hausted 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 

 Xewton, Boyle, and Hooke. Rumford and Young also 

 advocated the same view. Xevertheless, the general 

 opinion, even until the middle of the present century, 

 was that heat was due to the presence of a subtle fluid 

 known as ' caloric,' a theory which is now entirely 

 abandoned. 



