2o8 GREAT MEN OF SCIENCE 



obtuse angle, so that the two parts after reflection pass through 

 one another, and therefore are able to produce interference. 

 The experiment was completely successful in the manner ex- 

 pected. Interference bands were seen in the whole space 

 where the two reflections met. This mirror experiment 

 produced conviction on all sides regarding the wave nature 

 of light. It also resulted in Thomas Young's explanation 

 of Newton's rings being shown to correspond with 

 reality. 



Altogether, it was thenceforward necessary to regard a 

 ray of light as a wave propagated with the velocity of light as 

 measured by Roemer, a wave in a medium consisting in what 

 Huygens had already called ether, and not of the kinds of 

 atoms out of which, as was known since Dalton's and Berze- 

 lius' time, the ordinary matter of chemistry is made. This 

 result has been subsequently confirmed without exception, al- 

 though the ether itself is still full of questions. The lengths 

 of the waves could also be measured in the simplest and most 

 convincing manner by Fresnel's mirror experiments. The 

 exactitude could not, however, be driven in this case nearly as 

 far as with Fraunhofer's gratings, but the phenomena of 

 these, since they are more complicated and connected with 

 diffraction, are correspondingly less suited to provide 

 simple and direct insight. These wave-lengths are some- 

 what different for different colours, but very small through- 

 out the whole visible spectrum; they amount from the violet 

 to the red, only to from four to eight ten-thousandths of 

 a millimetre. 



Fresnel also devoted very thorough and important investi- 

 gations to polarised light and its production. Light of this 

 kind, having unsymmetrical properties, but not directly per- 

 ceptible to the eye as peculiar, had first been discovered by 

 Huygens, as resulting from double refraction, in which 

 'natural, unpolarised light' was divided into two polarised 

 rays. Not until more than a century later (1808) did Mai us 



