

THEORY OF LIGHT AND COLOURS. 1§3 



PROPOSITION V. 



When an Undulation is tranfmitted through a Surface terminating Prop. V. Law 

 different Medium*, it proceeds in fuch a Dire&Mb that the Sines f n ^?^ n of 

 of tlie Angles of Incidence and Refraclion are in the conjlant tranfmitted 

 Ratio of the Velocity of Propagation in the two Mediums. coXVfdif- 



(Barrovv, Left. Opt. II. p. 4. Huygens, delaLum. cap. 3. ferent mediums. 

 Euler, Conj. Phyf Phil. Tranf. for 1800, p. 123. Young's 

 Syllabus. Art. 382.) 



Corollary 1 . The fame demonftrations prove the equality 

 of the angles of reflection and incidence. 



Corollary 2. It appears from experiments on the refraction 

 of condenfed air, that the ratio of the difference of the fines 

 varies (imply as the denfity. Hence it follows, by Schol. I» 

 Prop. I. that the excefs of the denfity of the ethereal medium 

 is in the duplicate ratio of the denfity of the air; each particle 

 co-operating with its neighbours in attracting a greater portion 

 of it. 



PROPOSITION VI. 



When an Undulation falh on the Surface of a rarer Medium^ fo Prop. VI. Total 



obliquely that it cannot he regularly refracted, it is totally re-^ e ? e ^ lon / . of . un " 



/, . , , , I . . r • t ,™ -, diction falling 



fiected, at an Angle equal to tliat oj its Incidence,, (Phil, obliquely on the 



Tranf. for 1800, p. 128.) furface of a rarer 



medium. 

 Corollary. This phenomenon tends to prove the gradual 



increafe and diminution of denfity at the furface terminating 

 two mediums, as fuppofed in hypothefis IV; although Huy- 

 gens has attempted to explain it fomewhat differently. 



PROPOSITION VII. 



If equidiftant Undulations be fuppofed to pap through a Medium, Prop. VI. Un- 



of which the Parts are fufceptible of permanent Vibrations Come- dulat,ons are re * 

 i n i 7 r r j 7 • i • m • ,. , „ tarded by pafling 



what/lower than the Undulations, their Velocity will be fome- through a me- 



ivhat leffened by this vibratory Tendency ; and, in the fame dium vibrating 



Medium, tlie more, as the Undulations are more frequent. ^ency/ 



For, as often as the ftate of the undulation requires a change 



in the actual motion of the particle which tranfmits it, that 



change will be retarded by the propenfity of the particle to 



continue its motion fomewhat longer : and this retardation 



will be more frequent, and more conliderable, as the diffe- 



M 2 renc« 



