LIGHT. 



The preceding calculations are, as will be easily perceived, made onlv in 

 round numbers, with a view of rendering the principles of the investiga'tion 

 intelligible. In the table the exact results of the physical investigations which 

 have been carried on, on this subject, are given. 



In considering the two theories of light, each of which has been rendered 

 memorable by the eminent philosophers who have favored them respectively, 

 it is necessary that we should distinguish in each of them that which is purely 

 hypothetical, and which remains yet to be established as a matter of fact, from 

 that which expresses real and ascertained phenomena. 



In explaining these points, we cannot do better than adopt the clear and 

 candid language and reasoning of Sir John Herschel. In explaining gener- 

 ally the postulates of these theories, he says that in the corpuscular hypothesis 

 the following assumptions are made. 



1. That light consists of particles of matter possessed of inertia, and endued 

 with attractive and repulsive forces, and projected or emitted from all luminous 

 bodies with nearly the same velocity, of about two hundred thousand miles per 

 second. 



2. That these particles differ from each other by the intensity of the attrac- 

 tive and repulsive forces which reside in them, and in their relations to the 

 material world, and also ID their actual masses, or inertia. 



3. That these particles, impinging on the retina, stimulate and excite vision ; 

 the particles whose inertia is greatest producing the sensation of red, those 

 of the least inertia, violet, and those in which it is intermediate, the interme- 

 diate colors. 



4. That the molecules of material bodies and those of light exert a mutual 

 action on each other, which consists in attraction and repulsion, according to 

 some law or function of the distance between them ; that this law is such as to 

 admit perhaps of several alternations or changes from repulsive to attractive 

 force, but that when the distance is below a certain very small limit, it is 

 always attracted up to actual contact ; and that beyond this limit resides at 

 least one sphere of repulsion. This repulsive force is that which causes the 

 reflection of light at the external surfaces of dense media, and the interior at- 

 traction that which produces the refraction and interior reflection of light. 



5. That these forces havfc different absolute values or intensities, not only 

 for all different material bodies, but for every different species of the luminous 

 molecules, being of a nature analogous to chemical affinities or elective attrac- 

 tions ; and that hence arises the different refrangibilities of the rays of light. 



6. That the motion of a particle of light, under the influence of these forces 

 and its own velocity, is regulated by the same mechanical laws which govern 

 the motions of ordinary matter ; and that therefore each particle describes a 

 trajectory, capable of strict calculation, as soon as the forces which act on it 

 are assigned. 



7. That the distance between the molecules of material bodies is exceed- 

 ingly small in comparison with the extent of their spheres of attraction and 

 repulsion on the particles of light. 



