202 GREAT MEN OF SCIENCE 



metres in length, and having an objective about a quarter of a 

 metre in diameter. We know the parallax to-day of well over 

 one thousand stars; the one nearest to us, Proxima Centauri, 

 a weak companion of a Centauri, is distant 4.2 light years. 



In his last years Fraunhofer was especially busy with the 

 long-known phenomenon of diffraction. It is seen in a very 

 simple manner when light, for example from the sun, passes 

 through a very narrow opening into a dark room, and here 

 meets with an obstacle, for example a hair or a screen with a 

 small opening. We then find in the shadow of the obstacle a 

 distribution of light which shows clearly a departure from 

 straight line propagation: the light 'goes round the corner.* 

 This is the diffraction already fully described by Grimaldi.^ 

 Newton made further variation in the experimental investi- 

 gation of it, and thus provided a basis for interpreting the 

 phenomena, which are complex in their details; but Fresnel 

 was the first to reach a proper understanding of diffraction. 



Before Fresnel had completed his work, and devised 

 the mirror experiment which proved convincingly the 

 wave nature of light, Fraunhofer had made a great step for- 

 ward on the basis of the knowledge of Young's discussion, 

 a step which revealed an entirely new phenomenon, namely 

 the pure grating spectrum, which in contrast with the 

 minuteness and feeble luminosity of diffraction phenomena 

 hitherto known, showed a magnificent development of 

 colour, and opened up entirely new possibilities. In these 

 spectra, the various colour components of the light falling 

 on the grating, for example white light, appear adjacent to 

 one another, as in the spectrum of the prism, with the further 

 peculiarity, however, that they are arranged in exact accord- 

 ance with their wave-length, so that for example the extreme 

 red is deflected exactly twice as much as the extreme violet, 

 which has half its wave-length. Fraunhofer's first grating 



^ Grimaldi lived between 1618 and 1663 in Bologna. Newton begins the 

 third book of his Opticks with a mention of Grimaldi. 



