212 GREAT MEN OF SCIENCE 



light coming from the sun and artificial sources, likewise 

 operates transversely, but with extremely rapid change be- 

 tween all possible directions of oscillations at right angles to 

 the ray, while only a single such direction is present in the 

 case of polarised light. This supposition has always subse- 

 quently proved satisfactory. It also allowed Fresnel to 

 render the complicated phenomena of crystalline optics, 

 which Arago had in part observed, comprehensible in a per- 

 fectly satisfactory manner. 



Fresnel also succeeded in answering, in formulae which 

 still arouse our admiration to-day, the question of the inten- 

 sity of the reflected and transmitted light in ordinary refrac- 

 tion, together with the associated question of polarisation on 

 reflection and refraction. Finally, we must mention his 

 highly important calculation of the iight-drag,' that is an 

 influence to be expected of the motion of the material 

 medium (for example, water or glass) through which light 

 travels, upon the velocity of the latter. This calculation was 

 also shown to be correct, when measurements were made 

 first by Fizeau in 1853, in Paris, and later by others with 

 much greater refinement. It may astonish us that calcula- 

 tions, such as that of the intensity formulae or the light- 

 drag, should prove correct; for they treat the ether to a certain 

 extent as an elastic material body, which it is not, but with 

 which it has a certain similarity. But the question is simply 

 whether the suppositions go beyond the actually existing 

 similarity, and the success proves, that Fresnel had succeeded, 

 here also, in doing the right, thing, without doubt as a result of 

 his insight into the phenomena of light. 



Fresnel laid the complete foundations of the wave theory 

 of light, and also himself built upon them extensively. What 

 was added after his time was on the one hand the treatment 

 of special cases, for instance phenomena of diffraction, in 

 which F. M. Schwerd (schoolmaster in Speyer, 1792-1871) 

 did excellent work. This provided still further proof of the 



