34 KEPORT — 1881. 



positive, or natural, copies from one original picture. This process is the 

 foundation of all the methods now in use ; perhaps the greatest improve- 

 ments having been the nse of glass plates, first proposed by Sir John 

 Herschel ; of collodion, suggested by Le Grey, and practically used by 

 Archer ; and, more lately, of gelatine, the foundation of the sensitive film 

 now growing into general use in the ordinary dry-plate process. Not only 

 have a great variety of other beautiful processes been invented, but the 

 delicacy of the sensitive film has been immensely increased, with the 

 advantage, among others, of diminishing greatly the time necessary for 

 obtaining a picture, so that even an express train going at full speed can 

 now be taken. Indeed, with full sunlight ^^ of a second is enough, and 

 in photographing the sun itself (jo^oo 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 18.33 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 improved. 

 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 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. Rumford 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 subtle fluid known as 

 ' caloric,' a theory which is now entirely abandoned. 



Melloni, by the use of the electric pile, vastly increased our knowledge 

 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 vapours, both as regards the absorption and 

 radiation of heat. He proved, moreover, that the aqueous vapour 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 onr coal-beds — may have been due to 



