MODERN SCIENCE. 235 



fully displayed, furnished by ignited solids and liquids; (2) 

 band (or line) spectrum consisting of a number of bright 

 lines, and produced by ignited gases or vapours; (3) 

 absorption (or dark line) spectrum produced by the sun and 

 stars. Fraunhofer found an analytic formula as to the laws 

 of light, and invented several physical instruments, improving 

 lenses particularly. He found the haloes seen round the 

 sun and moon to be the diffraction of light by small globules 

 of fog in the atmosphere. But this admirable inquirer is 

 especially to be remembered as the FOUNDER OF SOLAR 

 PHYSICS. 



1787 1854. Ohm found THE LAW which bears his name : 

 " The intensity of an electric current is equal to the electromotive 

 force divided by the resistance " ; a law which enables us to 

 arrange a battery so as to obtain the greatest effect in any 

 given case. Another electric law was also discovered by 

 another German physicist, POGGENDORFF (17961874) : 

 " The electromotive force between any two metals is equal to 

 the sum of electromotive forces between all the intervening 

 metals." 



1788 1827. Fresnel modified Grimaldi's experiments on 

 the interference of light that is, the mutual action which two 

 luminous rays exercise upon each other and fully explained 

 the fundamental PHENOMENON OF INTERFERENCE, viz., that 

 the combined action of two pencils of red light forms a series 

 of parallel bands alternately red and dark ; if one ray is cut 

 off, the dark bands disappear. Fresnel rendered this a decisive 

 experiment (test experiment) of the truth of the UNDU- 

 LATORY THEORY OF LIGHT. For if light, which travels in 

 straight lines, were made of minute particles darting from the 

 sun and shot forward like bullets, there could not be alternate 

 dark bands, as the more particles there were, the more light 

 there should be ; whereas, if light is produced by the vibra- 

 tions of the luminiferous ether, as sound is produced by the 

 vibrations of air, the dark lines are at once explained. For, 

 the combination of two waves travelling the same distance at 

 the same rate will produce a band of light ; but two waves 

 travelling unequally will destroy each other when combining, 

 and thus cause a dark band. This remarkable fact, therefore, 



