50 MR. J. SMITH ON THE ORIGIN OF COLOUR 



different rays of light, but the writer just quoted seems 

 also to suppose that matter may modify them, for he adds : 

 " Lastly, as regards the colours generated by reflection of 

 light, it may be seen how it is possible that the velocity 

 of the incident wave may sustain a different modification 

 from different bodies, in consequence of which modifica- 

 tion of velocity different substances will be of different 

 colours."* 



72. One of the most acute and scientific writers of the 

 present day, a star of the first magnitude in the scientific 

 galaxy, when speaking of this subject says: "It will be 

 shown presently that the deviation of light by refraction is 

 a consequence of the difference of its velocities within and 

 without the refracting medium ; and when these velocities 

 are given the amount of refraction is also given. Hence it 

 would appear to follow unavoidably that rays of all colours 

 must be in all cases equally refracted ; and that therefore 

 there could exist no such phenomenon as dispersion. Dr. 

 Young has attempted to gloss over this difficulty by calling 

 to his aid the vibrations of the ponderable matter itself, as 

 modifying the velocity of the ethereal undulations within 

 it, and that differently according to their frequency ; and 

 thus producing a difference in the velocity of propagation 

 of the different colours; but to us it appears with more 

 ingenuity than success. "f I may remark that even this 

 supposition of Dr. Young's is not an original hypothesis, • 

 for Newton, in his 17th question at the close of his 

 Optics, says : " If a stone be thrown into stagnating 

 water, the waves excited thereby continue some time to 

 arise in the place where the stone fell into the water, and 

 are propagated from thence in concentric circles upon the 

 surface of the water to great distances . And the vibrations 

 or tremors excited in the air by percussion continue a little 



* Pescliel's Elements of Physics, p. 92. 

 t ISncy. Metropol. Article, " Light," p. 449. 



