:)() REPORT ON PHYSICAL OPTICS. 



to inquire whether an account can be given of it in the theory of 

 emission. 



The molecules of light cannot be supposed to exert any mutual 

 influence ; for the regularity of the laws of reflexion and refraction 

 compels us to consider them as independent, and each, separately, 

 the subject of those forces from which, in the theory of emission, 

 these laws are derived. The phenomenon of interference may, 

 however, be plausibly accounted for by the vibrations of the optic 

 nerve, produced by the impulse of the rays of light upon the re- 

 tina, and by the accordance or discordance of these vibrations 

 when caused by two interfering pencils. On this supposition, 

 which was suggested by Dr. Toung himself, the intensity of the 

 light will depend on the relation between the time of vibration of 

 the optic nerve and the interval of the impulses of the succeeding 

 particles. If this interval be equal to the time of vibration, or to 

 any multiple of it, the second impulse will add its effect to that of 

 the first, and the motion be accumulated. It will, on the other 

 hand, be destroyed, if the second impulse follows the first at an in- 

 terval equal to half that time. 



It is here assumed that the emitted particles succeed one 

 another at equal intervals, as will be the case if their emission be 

 owing (as Newton supposed it to be) to a vibratory motion of the 

 parts of the luminous body. But we must assume further that the 

 intervals of emission vary with the nature of the particles, in the 

 light of different colours ; or that all the red-making particles (to 

 use an expression of Newton) are emitted at one certain interval 

 all the blue-making at another and so for each different species of 

 simple light. Hence the vibrations of the parts of the luminous 

 body must be of different periods for the light of different colours. 

 This is, in truth, a part, and a necessary, part, of the theory of 

 waves ; but it has no connexion whatever with the principles of the 

 rival theory. 



II. Reflexion and Refraction of Light. 



To account for the phenomena of reflexion and refraction it is 

 supposed, in the Newtonian theory, that the particles of bodies and 

 those of light exert a mutual action ; that, when they are nearly 

 in contact, this action is attractive that at a distance a little 

 greater, the attractive force is changed into a repulsive one, and 

 that these attractive and repulsive forces succeed one another pro- 



