VII. ON THE 



THEORY OF LIGHT AND COLOURS. 



BY . 



THOMAS YOUNG, M. D. R R. S. 



PROFESSOR OF NATURAL PHILOSOPHY IN THE ROYAL INSTITUTION. 

 FROM THE PHILOSOPHICAL TRANSACTIONS. 



Read before the Royal Society, November 12, 1801. 



.A-LTHOTJGH the invention of plausible hy- 

 potheses, independent of any connexion 

 with experimental observations, caa be of 

 very little use in the promotion of natural 

 knowledge ; yet the discovery of simple and 

 uniform principles, by which a great num- 

 ber of apparently heterogeneous phenomena 

 are reduced to coherent and universal laws, 

 must ever be allowed to be of considerable 

 importance towards the improvement of the 

 human intellect; and in proportion as more 

 and more phenomena are found to agree with 

 any principles that are laid down, those prin- 

 ciples must be allowed to acquire a stronger 

 right to exchange the appellation of hypo- 

 theses for tliat of fundamental laws of nature. 

 The object of the present dissertation is 

 not so much to propose any opinions 

 which are absolutely new, as to refer some 

 theories, which have been already advanced, 

 to their original inventors; to support them 

 by additional evidence, and to apply them to 



a great number of diversified facts, which 

 have hitherto been buried in Obscurity, Nor 

 would it have been absolutely necessary, in 

 this instance, to produce a single new experi- 

 ment; for of experiments there is already an 

 ample store, which are so much the more un- 

 exceptionable, as they must have been con- 

 ducted without the least partiality for the 

 system by which they will be explained ; yet 

 some facts, hitherto unobserved, will be 

 brought forwards, in order to show the per- 

 fect agreement of that system w ith the multi- 

 farious phenomena of nature, which are 

 connected with it. 



The optical observations of Newton are yet 

 unrivalled ; and, excepting some casual inac- 

 curacies, they only rise in our estimation, as 

 we compare them with later attempts to im- 

 prove on them. A further consideration of 

 the colours of thin plates, as they are de- 

 scribed in the second book of Newton's op- 

 tics, has converted that prepossession which 



