ON THE THEORr OF LIGHT AVD COLOims. 



625 



ihoB a reflection is always secondarily pro- 

 duced, wiien the ra)'S of light pass from a 

 denserto a rarer stratum." (Phil. Trans. 1800. 

 127.) But it is not necessary to suppose an 

 attraction in the latter case, since the effort 

 to proceed would be propagated backwards 

 without it, and the undulation would be re- 

 versed, a rarefaction returning in place of 

 a condensation ; and this will perhaps be 

 found most consistent with the phenomena. 



Proposition v. When an Undulation 

 is transmitted through a Surface terminating 

 different Mediums, it proceeds in suck a 

 Direction, that the Sines of the Angles of 

 Incidence and Refraction are in the con- 

 stant Raiio of the Felocity of Propagation in 

 tlie two Mediums. 



(Barrow Lect. Opt. II. 4. Huygens de 

 la Lum. cap. 3. Euler Conj. Phys. Phil. 

 Trans. 1800. 128. Young's Syllabus, Art. 

 582.) 



Corollary 1. The same demonstration 

 prove the equality of the angles of reflection 

 and incidence. 



Scholium. It appears from experiments 

 on the refraction of condensed air, that the 

 difference of the sines varies simply as the 

 density. And the same is probably true in 

 other similar cases. 



Proposition vi. When an Undulation 

 falls on the Surface of a rarer Medium, so 

 obliqueli/ tltat it cannot be regularly refracted, 

 it is totally reflected, at an Angle equal to 

 that of its Incidence. 



This phenomenon appears to favour the 

 supposition of a gradual increase and di- 

 minution of density at the surface terminat- 

 ing two mediums, (Phil. Trans. 1800, 128.) 

 although Hwygens has attempted to ex- 

 plain it somewhat differently. The velocity, 

 with which the successive parts of the 



undulation arris'e at the reflecting sur- 

 face, is sufficient to determine the angle of 

 reflection ; in the same manner as when a 

 bird is swimming in a stagnant piece of wa- 

 ter, we see a rectilinear wave diverging at a 

 certain ano;le on each side. The total re- 

 flection seems to require tiie assistance of 

 the particles of the rarer medium, to which 

 the motion of the preceding portion of the 

 undulation has been partly communicated, 

 without being able to produce any other ef- 

 fect than that of urging them in the direc- 

 tion of the surface, and enabling them to re- 

 sist the force of the direct undulation, which 

 tends to remove them from the surface. 



Proposition vit. If equidistant Undula- 

 tions be supposed to pass through a Medium, 

 of which the Parts are susceptible of perma- 

 nent vibrations somewhat slower than (he Un- 

 dulations, their Velocity will be somewliat 

 lessened by this vibratory Tendency ; and, in 

 the same Medium, the. more, as the Undula- 

 tions are more frequent. 



For, as often is the state of the undulation 

 requires a change in the actual motion of the 

 particle which transmits it, that change will 

 be retarded by the propensity of the particle 

 to continue its motion somewhat longer : and 

 this retardation will be more frequent, and 

 more considerable, as the difference between 

 the periods of the undulation and of the -na- 

 tural vibration is greater. 



Corollary. It was long an established 

 opinion, that heat consists in vibrations of 

 the particles of bodies, and is capable of be- 

 ing transmitted by undulations through an 

 apparent vacuum. (Newt. Opt. Qu. 18.) 

 'this opinion has been of late very much 

 abandoned. Count llumford and Mr. Davy, 

 are almost the only modern authors who have 

 appeared to favour it; but it seems to have 



