201 Adronomic^il and Nautical Collections. 



of Newton ; who first demonstrated that the effect produced 

 by white light depended always on the union of the different 

 effects of the coloured rays of which it is composed. 



Of Reflection and Refraction. 

 It has been shown, from the example of the collision of 

 elastic bodies, in what manner a part of the undulatory mo- 

 tion must be reflected at the surface of contact of two me« 

 diums of different densities : while another part is transmitted, 

 and propagated in the second medium : and for further in- 

 formation on this subject, the reader may consult the elegant 

 Memoir of Mr. Poisson, on the reflection of undulations at 

 the surface of contact of two elastic mediums of different 

 densities, in which the general proposition is rigorously de- 

 monstrated. But although the division of the light, into re- 

 flected and transmitted rays, has been thus explained, the 

 laws, by which their directions are governed, have not yet 

 been demonstrated : and it will now be proper to attempt 

 such a demonstration, from the simplest possible considera- 

 tions, omitting all complicated development, which might be 

 required for a general and strictly mathematical view of the 

 question, in order to make the subject as clear and intelligi- 

 ble as possible. 



D G' a- ii 



Let ED and FG be two incident rays, proceeding from the 

 same centre of undulation, which we may suppose to be at 

 an infinite distance, so that they may be parallel to each other : 

 let AB be the reflecting surface ; let Gl be drawn perpen- 

 dicular to FG and ED : then GI will represent the surface of 

 the wave at the moment when it meets the reflecting surface 

 in G. Now, according to the principle of Huygens, we may 

 consider each of the points agitated by the wave, as G and 

 D, in the light of new centres of agitation acting indepen- 

 dently, and emitting rays in an infinity of directions, and with 



