236 PROGRESS OF SCIENCE. 



is explained in the most satisfactory manner by the wave- 

 theory of light, and itself serves to demonstrate the wave- 

 theory. From that moment then the undulatory doctrine 

 was established, one of the finest demonstrations in the field 

 of science. Fresnel and Young, who worked one part of the 

 problem independently, at once accounted for a large number 

 of optical phenomena. Fresnel showed those of polarisation 

 are explicable by the wave-theory; further, by the mere 

 manipulation of mathematical symbols, he was able by the 

 light afforded by the same theory to foresee several compli- 

 cated phenomena, and explain those of reflection and 

 refraction. In these investigations he had a powerful colla- 

 borator in Arago, who is equally celebrated as a physicist and 

 an astronomer, as we have just seen. Fresnel, besides this 

 grand work, made powerful LENSES for LIGHTHOUSES; 

 invented a RHOMB, which bears his name, for converting 

 plane into circular polarised light ; by the same instrument 

 light can also be polarised elliptically. 



I ySS 1878. Becquerel (Antoine Cesar) made a thermo- 

 electric battery, an apparatus which has caused rapid advances 

 in heat; was one of the founders of ELECTROCHEMISTRY; 

 made numerous and successful researches on thermo-electric 

 phenomena; invented the electric PYROMETER; also an 

 ELECTRIC THERMOMETER used to ascertain the temperature 

 (i) of plants and animals ; (2) of the earth at different depths ; 

 (3) of the air at different heights. 



1791 1867. Faraday, one of the most eminent men of 

 science of our age, immensely contributed to the progress of 

 electricity by his experimental researches; discovered the 

 mutual ROTATION OF ELECTRIC WIRES AND MAGNETS a 

 conclusive proof of the connection between magnetic and 

 electric currents ; reversely to Ampere, he obtained 

 ELECTRICITY WITH MAGNETS a discovery soon to become 

 practically useful in electric telegraphy ; showed the action of 

 magnets on light a phenomenon of high importance. In 

 connection with Coulomb's discovery (of attraction and 

 repulsion), Faraday found (1845) tnat a ^ solids and 

 liquids are either attracted or repelled by a powerful electro- 

 magnet. Those which are attracted he called magnetic 



