Reflection and Refraction of Light. \ 43 



2. That the particles of the ether are set in motion by tlie agitations of 

 ponderable matter, and the communicated motions are propagated to the 

 adjacent ether continually in all directions, according to the laws which 

 regulate undulations excited in other media. 



3. That the elasticity of the ether is less in bodies than in spaces void of 

 other matter, and less in bodies which have a greater, than in those which 

 have a less refractive density. 



4. That the vibrations of the ether are propagated througli bodies by 

 means of the ether in their interior, with a less velocity in bodies which 

 have a greater refractive density. 



5. That the vibrations of the ether agitating the nerves of the eye excite 

 vision. 



6. That the frequency of the pulses, or the number acting on the eye in 

 a given time, determines the colour; and the extent, or amplitude of the 

 vibrations, determines the intensity or brightness of the light. See Sir J. 

 Herschel's Treatise on Light, art. 563. 



7. (III.) It remains to exhibit the basis on which the third explanation is 

 founded. 



Natural philosophy does not any where inform us what is the connecting 

 link between matter and mind ; or in what manner any sort, or state, or 

 situation of matter, serves to produce this or that idea : we can trace its 

 progress to a certain point, but cannot follow it into the chamber of imagery. 

 Hence when we pursue the subject up to sensation, we must rest ultimately 

 on the simple fact, as the limit of our knowledge : the fact however proves 

 an inscrutable connection between mind and matter, established by the 

 power and will of the Creator. Therefore in every system or theory, the 

 sensations of light and of colour must be attributed to the actions of lumin- 

 ous particles, which follow, after we have traced their progress to the 

 organs of vision. 



Hence the third principle in the Newtonian, and the fifth in the Huy- 

 genian theory must be freely admitted, and similar conclusions must in 

 every case be granted : in respect of the emission and motion of luminous 

 atoms, the present views agree with those of Newton ; the great diflference 

 is in the constitution of the atoms, which therefore requires a particular 

 explication. 



8. According to Newton, these are very minute solids, massy, hard, 

 impenetrable, and moveable ; and this notion, although hypothetical, is 

 generally received. Newton does not consider gravity as an essential 

 property of matter, but as a power or virtue appended to every particle. 

 The name indicates a property, but if a property, it necessarily inheres in a 

 substance co-cxtensive with itself. Of the minute solids and the multitude 

 of attractions and repulsions which have been imagined, wc are still more 

 ignorant, we do not know even that they exist ; arc they not the fictions 

 of fancy ? worse than useless in philosopiiy, unless employed confessedly 



