2o6 GREAT MEN OF SCIENCE 



arises in the case of diffraction as a result of differences in 

 path length from the various illuminated points of the open- 

 ing producing diffraction, to the screen; half a wave-length 

 difference in the path always gives opposite states of vibra- 

 tion and hence darkness, or destruction of the light at the 

 point in question of the receiving screen. The colours 

 arise from the difference in the wave-length of the lights of 

 different colour. If, for example, red is blotted out of white 

 light, the remainder appears green, as had been clear since 

 Newton's study of colour. In this way, the colour phe- 

 nomena even allowed the wave-lengths of the lights of 

 various colour to be calculated. 



Something of this kind had already been attempted, before 

 Fresnel's time, by Thomas Young, in the case of Newton's 

 rings, which he interpreted as a phenomenon of inter- 

 ference, Newton's measurements only needing to be suitably 

 interpreted in order to allow the wave-lengths to be deduced. 

 Young had also interpreted the diminution in the size of the 

 rings, obtained by Newton when he filled the space between 

 his glasses with water, as a reduction of the wave-length in 

 water. He had also concluded from the diminution in 

 wave-length, according to the ideas already developed by 

 Newton, that the velocity of light is smaller in water, and he 

 had shown from Newton's figures that this diminution 

 exactly corresponds in amount to Huygens' explanation of 

 the refraction of waves, so that everything thus corresponds 

 in the best possible way with the assumption of the wave 

 nature of light. Young had also been able to explain the 

 black spot in the middle of Newton's rings, the cause of 

 which is not immediately obvious, by means of an acoustic 

 analogy, and had tested this explanation by special and 

 finely devised experiments, and confirmed it.^ 



^ A good and well-informed critical discussion of the achievements of 

 Young, and also of those of Fresnel, Grimaldi, and Malus, is to be found 

 in E. Mach's Physical Optics. 



