474 HISTORY OF SCIENCE. 



upon each other. But if one of the bundles was turned round the 

 incident ray so that the planes of polarization of the light from the two 

 slits were perpendicular to each other as, for example, when one 

 bundle was inclined from above downwards, and the other from right 

 to left then no interference bands whatever were produced. These 

 following results were thus conclusively established : 



FIG. 216. FRANCOIS ARAGO. 



I st. Two rays of light polarized in the same direction act upon each 

 other like rays of ordinary light, the phenomena being absolutely the same 

 in both cases. 



2nd. In the same circumstances under which two rays of ordinary 

 light appear to destroy each other, two rays polarized in perpendicular 

 directions have on each other no appreciable action. 



FRANCOIS ARAGO (1786 1853), the distinguished French savant 

 whose name has been so often mentioned in this chapter, was another 

 of those pupils of the Ecole Polytechnique who made that college 

 famous by their brilliant scientific careers. Arago succeeded Monge 

 in the professorship of geometrical analysis, and he became eminent 

 not only as a mathematician and astronomer, but also as a physicist. 

 Many of his discoveries and researches in connection with light and 

 with electricity are of great interest and importance. He published a 

 popular treatise on astronomy, and other writings which are highly 

 esteemed. 



We have now described at length some of the phenomena upon which 

 the undulatory theory of light depends, and throughout we have been 

 more anxious that the reader should clearly realize a few of the more 

 important facts and the inferences from them, than that he should be 



