142 Reflection and Refraction of Light. 



planations of all the leading facts in this delightful and extensive science, 

 on different principles, principles already admitted and established in 

 other departments of philosophy. 



4. In order to do justice to the subject, and place it fairly before the 

 reader, so as to enable him readily to enter into a view of the cause, or 

 supposed cause of reflection and refraction, according to the three notions in 

 question, it will be proper, or rather necessary, to present a brief statement 

 of the principles on which each system is built. 



5. (I.) The Newtonian or corpuscular theory of light requires the fol- 

 lowing postulates : — 



1- That light consists of solid particles having inertia, and some definite 

 magnitude ; and that they are projected from self-luminous bodies with 

 nearly equal velocities. 



2. That these atoms of light differ from each other in their actual masses 

 or inertia, and in the intensity of their attractive and repulsive forces, and 

 in their relations to other bodies. 



3. That these particles, impinging on the eye, excite vision j that the 

 different sorts produce the sensation of different colours, and that they are 

 refracted, some more than others. 



■I- That there is a mutual action of the particles of light, and those of 

 other bodies, on each other; the actions being alternately attractive and 

 repulsive. 



5. That these forces have different absolute values for the different sorts 

 of bodies, of a nature analogous to chemical affinities. 



6. That these attractive and repulsive forces, though indefinitely greater 

 than gravitation, are absolutely insensible at all measurable or appreciable 

 distances from the particles which exert them ; and yet these exceedingly 

 small distances are very large, in comparison with the distances between the 

 particles of the reflecting or refracting media. 



7. That every luminous particle during the whole of its progressive 

 motion is continually passing through periodical changes, or alternations, 

 called fits of easy reflection, and easy transmission ; being more easily re- 

 flected in the former state, and transmitted in the latter. See Sir J. 

 Herschel's Treatise on Light, Ency. Metrop. art. 526. 



6. (II.) The Huygenian, or undulatory theory, requires, with some 

 others, the following postulates. 



1. That an elastic ethereal fluid fills all space, and pervades the inter- 

 stices of all bodies, having inertia but not gravity ; and presenting no ob- 

 servable resistance to tlie motions of planets and comets.* 



* Encke's comet is said to have had its period shortened, an indication that it has 

 suffered some resistance, but if so it may arise from the solar atmosphere affecting it 

 when near the periiielion ; for although the gross atmosphere of the earth extends 

 only to an altitude of forty or fifty miles, yet the ethereal matter enveloping it may 

 extend to some hundreds, or even thousands of miles, and that of the sun more in ' 

 proportion. 



