MODERN SCIENCE. 229 



ditions of the problem as stated by Young, seems to have 

 arrived first at the correct solution with the assistance of 

 Arago. Still, it was only after he had communicated his 

 results to Young that decisive experiments were made, and 

 led to the complete settlement of the question (1816) so 

 that their names cannot be dissociated in reference to it. The 

 undulatory theory once established, Young showed that the 

 phenomena of diffraction are to be explained by it. The 

 wave theory also reveals an intimate connection between 

 light phenomena and heat phenomena ; it also shows how 

 completely analogous the phenomena of light are to those of 

 sound ; it readily explains the colours of different bodies 

 opals, mother-of-pearl, butterflies' wings, the feathers of birds, 

 soap-bubbles amongst others. For according to this theory, 

 " certain bodies have the property of exciting undulations of 

 different lengths, and therefore of producing light of given 

 colours " : white light or daylight results from the co-existence 

 of undulations (waves) of all possible lengths. The theory like- 

 wise explains the colours of all transparent bodies, as also the 

 colours of the spectrum, and their alteration according to 

 distance. All these phenomena then depend upon wave- 

 lengths, and these are infinitely varied. They have been 

 calculated to range in parts of an inch from 



Extreme Red 0*0000266 to Extreme Violet 0-0000167 ; 

 whilst according to Sir John Herschel's mathematical 

 investigation, the number of undulations in an inch range 

 from 



Extreme Red 37,640 to Extreme Violet 59,750; 

 and the number of undulations in a second range from 



Extreme ^^458,000,000 of millions, to 



Extreme Violet 727,000,000 of millions. 



But returning to Young, besides the part he took in 

 establishing the undulatory theory, he also laboured to de- 

 monstrate the existence in interstellar space of A LUMINI- 

 FEROUS ETHER a hypothesis entertained by Huygens, and 

 almost demonstrated by Encke since, as we have seen a 

 hypothesis, so far, which is, however, pretty generally accepted. 

 This ether " more solid and elastic than steel," and which 

 must exist if the undulatory theory is true, exerts a pressure 



