•aaa On the unequal RefJexthUlty of Light. 



anceftors could even difcover its ruins; that their prefent church was built to fupply the place 

 of that which the waves wafhed away ; and that even their prefent clock belonged to their old 

 church. So many concomitant, though weak teftimonies, incline me to believe their report, 

 and to fuppofe that fome of the ftormy inundations of the North Sea, which, in thefe laft 

 centuries, have waflied away fuch large tradls of land on its (hores, took away a foil refling 

 on clay, and at laft uncovered the frees which are the fubjedl of this paper. 



IX. 



Optical Remarks^ chief y relating to the Refexibility of the Rays of Light *. By P. pREFOSTy 

 Pfofcjfor of Philojophy at Geneva, of the Academy of Berlin, of the Society des Curieux de 

 la Nature, and of the Royal Society of Edinburgh. 



X ART I. Concerning reflexibility. Seflicn i. The word reflexibility is ufed in two 

 different fenfes. 



I. Newton f indicates, by this word, that property of a ray of homogeneous light, by 

 virtue of which this ray is reflefled, if it fall under a certain angle of incidence ; and tranf- 

 mittcd, if it fall under a certain fmaller angle: or, more fimply, a difpofition to be reflefted, 

 and not tranfmitted at the limit which feparates two refraiSling mediums J. 



This philofopher thinks that the reflexibility of the rays of light, taken in this fenfe, is not 

 the fame in all the feveral kinds. He eflabiifhes, by experiments, which he concludes to be 

 decifive, that the moft refrangible rays are alfo the moJl: refle.-.ible. So that, according to him, 

 all other circumftances being the fame, if a white ray fall under a certain angle on the dircdt- 

 ing furface, the violet ray will be refleiied, while the fix others will be ftill tranfmitted and 

 refra£ted. But by augmenting the angle of incidence,, the fucceillve refle6lion of all the rays 

 will be obtained from the violet, which is the moft reflexible, to the red, which is the 



leaft fo. 



Mr. Brougham § does not find the experiments of Newton, by which he eftablilhes this 

 propofition, conclufive: and, upon the foundation of another experiment, he eftablilhes the 

 contrary propoiition ; namely, that all the rays have the fame difpofition to be refledted, pro- 

 vided the angle of incidence be the fame. 



2. Mr. Brougham underftands by reflexibility, a difpofition in the ray to be reflected 

 nearer to the perpendicular, to a certain degree ; or, in other words, a property of the 

 homogeneous ray,_ by which its angle of reflexion bears a certain ratio to its angle of 

 incidence, which is not the ratio of equality, except in certain cafes, which he points out. 



According to this philofopher, the above ratio varies in each homogeneous ray. The ratio 



* Tranflated from the French original, in the Philof. Tranf. 1798. 



f Opt. 1. i. part i. prop. 3. X Qpt. 1. i. part i. defin. ^, 



§ PUilof. Tranf. 1796. p. 471. or Philof. Journal, i. 595. 



