48 ESSAYS AND OBSERVATIONS 



according to the laws of our conftitutlon, thd 

 ideas of different colours may be excited ^; iii 

 Jike manner as the ideas of different tones 

 arife from different vibrations of the air com- 

 tnunicated to the auditory- organ. It has 

 been faid, That the different fenfations ex- 

 cited in the mind cannot arife from the dif- 

 ferent force of the particles of light; fince the 

 colour of homogencal rays is not altered 

 by paffing thro' different media^ tho' their 

 velocity be thereby always increafed or di- 

 minifhed -j-. But it ought to be confidered, 

 that every ray, as it muft pafs at laft thro'' 

 the humours of the eye in order to vifion, 

 falls upon \S\t retina with one given velocity, 

 whatever number of refra<flions it has pre- 

 vioufly undergone : for the velocity of any 

 ray in any one medium being, to its velocity in 

 any other medium^ in a conftant proportion, 

 mz. the inverfe of the fines of incidence and 

 refradiion, when a ray paffes from the one 

 into the other ; it is manifeft, that each ray 

 muft have a certain determined velocity in 

 any given med.um^ which cannot be either 

 increafed or diminifhed by making the ray 



paft 



* Ne'ujton'-s Optics, query 13. 



f Mujfchenbrceck, Elementa Phyfices, § 116 J. 



