I64f 



PROPERTIES OF LIGHT. 



axis c' is proportional to the square of the cosine of the same 

 angle. 



When the metallic mirror makes equal angles with the 

 axes b and f, b' makes an angle of 45" with each ; and then 

 the light comports itself in the same manuer on a glass pa- 

 rallel to the axis b', or to tlve axis c'; it seems, in this case, 

 to have resumed all the characters of direct light. 

 Ray from the If the ray reflected by the metuUic mirror be dissected 

 d^ss^ d"b'-'^°'^ by means of a crystal of calcareous spar, in disposing its 

 calcareous principal section parallel to the plane of reflection, the pro- 

 ^^"'^' portion of the intensities of the ray refracted extraordinarily 



and the ordinary ray is equal to the square of the tangent of 

 the angle included between the two axes, b, b'. 

 Light reflected If the light be made to undergo several reflections from 

 froni'^metallV' ™^t^""^ mirrors, before subjecting it to the action of a second 

 mirrors. transparent body, the phenomena are analogous to those I 



have mentioned. If the axis b' of the second ray be parallel 

 to the axis Z> or c of the first; if the axis b" of the third 

 be parallel to the axis b' or c of the second; and so for the 

 rest; the property of the light already laid down will be in 

 no respect altered : but if these axes be inclined to one 

 another, it will be divided with respect to the two con- 

 secutive mirrors, according to the law I have mentioned., 

 Reflected liglit If the surface of a polished opake substance, as black 



received on garble, be made to turn round the axis c of the first re- 

 black marble. i ,• i -.i i i- • • i 



fleeted ray, the reflected hght will be seen to dimmish to a 



certain point, at which it is null, and beyond which it begins 

 to increase. 

 Ordinary phe- All the ordinary phenomena of optics may be explained 

 lomenaof op- eit},er on the hypothesis of Fluyghens, who supposed them 

 onMve'^h'vpo ^ to be produced by the vibrations of an ethereal fluid; or 

 thesi? of Huy- agreeably to the opinion of Newton, who supposed them 

 Newton*:' ° ^^ be produced by the action of bodies on luminous parti- 

 cles, considered as themselves belonging to a substance 

 obeying the attractive and repulsive powers, that serve to 

 explain other physical phenomena. The laws respectiag 

 the course of rays in double refraction too may be explained 

 buf tho e here.on either hypothesis. But the observations I have related 

 "ecSaU ""* 'prove, that the pheaoxnena of reflection are different at the 

 I •« Bame 



