REPORT ON PHYSICAL OPTICS. 305 



regarded as a weighty argument in favour of that theory. It now 

 remains to inquire whether any 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 phe- 

 nomenon 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 retina; and by the 

 accordance or discordance of these vibrations when caused by 

 two interfering pencils. On this supposition, which was sug- 

 gested by Dr. Young himself, the intensity of the light will de- 

 pend on the relation between the time of vibration of the optic 

 nerve, and the interval of the impulses of the succeeding parti- 

 cles. 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 

 interval 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 par- 

 ticles, 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 probably for many alternations. The absolute values, 

 or intensities, of these forces are different in different bodies : 

 1834. X 



