122 Britain's Heritage of Science 



then quite novel ; it marked a considerable step in the 

 philosophical treatment of the subject. As one result of this 

 investigation, the complete internal reflexion which occurs 

 when sound passes from one medium to another, possessing 

 different elastic properties, was demonstrated in opposition to 

 Cauchy, who had come to a contrary conclusion. 



The subject of light is dealt with in a masterly manner 

 in two papers. The general properties of elastic media are, 

 for the first time, examined mathematically, and light is 

 treated as a special case of waves passing through a perfectly 

 elastic body. Green must be considered to be after Newton 

 the founder of the Cambridge school of Mathematical 

 Physics. He did not like Cauchy and Franz Neumann 

 discuss the causes which give bodies their elastic properties, 

 and could, therefore, dispense with any hypothesis on the 

 mutual action of molecules, or on the ultimate constitution 

 of the luminiferous aether. All he needed was the assumption 

 that its properties were such as to comply with the principle 

 of the conservation of energy. That principle had not, at 

 that time, been formulated, but appears implicitly in Green's 

 work. The investigation solved, under certain suppositions, 

 the problem of the transmission, reflexion and refraction of 

 waves passing through homogeneous elastic bodies. The only 

 question that remained was, whether the observed laws of 

 light could be made to agree with the mathematical formulae 

 obtained. The two main experimental tests that could be 

 applied were the intensities of light reflected at the surface 

 of transparent bodies and the laws of double refraction. 

 The French physicist Fresnel had broken the ground, and 

 obtained satisfactory solutions for both problems, but his 

 analysis was not free from serious defects, and the hypothesis 

 he applied in one case was inconsistent with that introduced 

 in the other. The more rigid treatment of Green, together 

 with the subsequent investigations of Stokes, McCullagh and 

 Rayleigh, led to a deadlock, for no consistent hypothesis 

 could be framed to fit all cases. Fortunately Clerk Maxwell's 

 electrodynamic theory of light disposed of these difficulties. 



Green's first paper on Electricity -and Magnetism is 

 considered to be his most important contribution to science, 

 but being of a highly technical character, it must suffice to 



