152 PROGRESS OF SCIENCE IN THE CENTURY. 



historically justifiable to regard Newton as the 

 founder or even upholder of the emission-theory.* 



The ray of light, on the emission-theory, was sim- 

 ply the trajectory of a particle in rectilinear motion ; 

 the ray of light, as Newton described it, possesses a 

 regular periodic structure, and the period or interval 

 of fits characterises the colour of the ray. This was 

 an important result. It only required a fitter inter- 

 pretation to transform the luminous ray into a 

 vibratory wave, but for this there was a century to 

 wait, and Dr. Thomas Young, in 1801, had the 

 honour of discovering it.f 



The Wave-Theory of Young. — Thomas Young 

 (1Y73-1829), whose precocious genius, persisting in 

 manhood, remained, as Tyndall says, '' hidden from 

 the appreciative intellect of his countrymen," was led 

 from a study of the eye and its optical properties, to 

 an enquiry into the phenomena of thin plates and 

 " interference," and in the course of this he rehabili- 

 tated the undulatory theory (1801), published in the 

 Philosophical Transactions for 1802. 



The theory is, in general terms, that light consists 

 of vibrations in an all-pervading elastic ether, and 

 that the vibrations, unlike those of sound, are in di- 

 rections at right angles to the direction of propaga- 

 tion. So far as Young went, the theory was, in 

 simple language, that a homogeneous ray of light is 

 analogous to the wave produced by a musical sound, 

 and that the vibrations of light ought to compose or 

 interfere, like those of sound. " But his hypothesis 

 found no favour; his principle of interference led 



♦A. Cornu, The Rede Lecture: "The Wave Theory of 

 Light: its influence on Modern Physics," Nature, July 27, 

 1899, pp. 292-297, 



t From Prof. Cornu's Rede Lecture. 



