ACTION OF GOLD ON LIGHT. 173 



which consists of all the colours, is consequently a mixture of 

 waves of all lengths between the limits of the extreme red and 

 violet. The determination of these minute portions of time and 

 space, both of which have a real existence, being the actual 

 results of measurement, do as much honour to the genius of 

 Newton as that of the law of gravitation. 



The number of advancing waves of light in an inch is known 

 to be from 37,600 to 59,880, and the ntimber of lateral vibrations 

 is from 458 to 727 billions in a second, but the extent of these 

 lateral vibrations of the particles of the ethereal medium is not 

 known, though both their extent and velocity are probably very 

 small compared with the length of the advancing waves and the 

 velocity of propagation. Colour is identified with the number of 

 vibrations ; but whether reflection, refraction, absorption, &c., 

 have any relations to the lateral vibrations, or whether they are 

 dependent in part upon some physical action of the ethereal 

 medium unknown and unsuspected, are points as yet undeter- 

 mined. To ascertain these circumstances, Dr. Faraday instituted 

 a series of the most refined experiments upon the relation of the 

 minute particles of metals to the vibrations of light. 



Gold acts powerfully on light, and possesses a real transparency, 

 transmitting green rays when very thin ; and being capable of 

 extreme division by solvents without losing its metallic charac- 

 ter, its particles transmit rays of various colours according to 

 their size ; those that transmit the rose-colour in Bohemian glass 

 are of inconceivable minuteness. The progressive waves of the 

 ether are so long compared with the dimensions of the molecules 

 to which gold can be reduced, that it seemed probable to Dr. 

 Faraday when the latter were placed in a sunbeam that some 

 effective relation might be discovered between them and the 

 smaller vibrations of the ethereal medium ; in which case, if 

 reflection, refraction, &c., depended upon such relations, there 

 was reason to expect that these functions would change sensibly 

 by the substitution of different sized particles of the gold for one 

 another. At one time Dr. Faraday hoped he had changed one 

 colour into another by means of gold, which would have been 

 equivalent to a change in the number of vibrations ; but although 

 he has not yet confirmed that result, his researches are of the 

 greatest interest.* 



* Bakerian Lecture, by Michael Faraday, Esq. Phil. Trans. 1857. 



