NE-W PROPERTY OF REFLECTED LIGHT. IQj 



fraction through the crystal, will produce four pentib, 

 which will follow precisieiy the course of the reflected rays-. 

 Thus the law of the double refraction being known, that of 

 the double reflection may easily be deduced from it. 



I shall no\y proceed to that kind of phenomenon, which Quantity and 

 is the subject of this paper ; and which relates, not to the i^i'^rties of 

 law according to which the rays are directed, but to the 

 quantity and properties of the light they coatain. 



Let us suppose the angle of incidence to be constant, 

 and the crystal placed horizontally. If we turn the rhorn- 

 boid round th.e perpendicular, so as to approxitnate its priur 

 cipal section to the incident rays, we shall perceive a gr^- 

 4ual diminution of intensity in the ordinary ray reflected 

 extraordinarily, and of the extraordinary ray reflected ordi- 

 liarily. In fine, when the plane of the principal section 

 coincides with the incident ray, these two reflected rays 

 disappear entirely, and nothing remains but the ordinary 

 ray reflected ordinarily, and the extraordinary ray reflected 

 extraordiiiarily. The latter however has much less intensity 

 than the former. 



If now, the incident ray continuing to he inclvided in 

 the principal section, we increase or diminish the angle of 

 incidence, till it becomes 56° 30', the latter reflected ray 

 will disappear altogether ; and only that, which has been, 

 ^•efracted ordinarily, and reflected ordinarily, will remain. 

 Beyond or within this angle, the extraordiiiary ray reflected 

 extraordinarily will reappear with an intensity proportional 

 to the remoteness from this anajle. The angle of incidence 

 I have mentioned is that, under which a ray reflected at the ^^' 



first surface would have acquired the property of being di- 

 vided into two pencils, or ren^aining in one, as takes place 

 Ht the surface of any other transparent body. The pre- 

 ceding phenomenon may easily be connected with the expe- 

 riment, \x\ which water was taken for an example: for if we 

 let fall on the surface of the rhomboid, under an angle of 

 5^ 30', or thereabout, a ray disposed to be refracted only 

 in one extraordinary pencil, this ray will produce no partial 

 reflection at the first surface; which seems to explain, why 

 it produces none'at the second, 



However, it is not the sqme, wheri the plane of incidence 



make^ 



