174 Astronomical and Nautical Collections. 



Brewster. We are not yet certain whether this law is 

 rigorously correct, or merely an approximation ; but the lat- 

 ter supposition seems to be the most probable. 



At other incidences, the polarisation is only partial ; that 

 is to say, in turning the rhomboid round, the image never 

 wholly disappears. The images vary indeed in brightness, 

 but their minima, which always correspond to the directions 

 of the principal sections, do not become equal to nothing. 

 In short, when the incident rays are perpendicular, or nearly 

 parallel to the surface, the reflected light no longer exhibits 

 any traces of polarisation ; that is to say, the two images are 

 always of equal intensity in every position of the rhomboid. 



Many opaque bodies, which are not too highly refractive, 

 such as marble, and black varnishes, are capable of completely 

 polarising the rays which are regularly reflected at their sur- 

 face ; while other bodies perfectly transparent or semitrans- 

 parent, but highly refractive, such as diamond and glass of 

 antimony, never polarise it completely. But the metals are 

 the least capable of polarising the light which they reflect, 

 even in the most favourable circumstances. It is to be re- 

 marked, that the incidences, which correspond to the maximum 

 of polarisation, approach so much the more to the surface as 

 the reflective body is more refractive ; if at least we may 

 judge by the abundance of light reflected, when the body is 

 completely opaque, like the metals. 



Transparent bodies do not polarise light by reflection only, 

 but by refraction also, and the more completely as their sur- 

 face is the more inclined to the rays ; but it is never com- 

 pletely polarised in this manner, unless it is caused to pass 

 through several polarised plates in succession : and so many 

 the more plates are required as they are the less inclined to 

 the incident rays. Ma.lus, to whom we are also indebted 

 for the discovery of this mode of polarisation, demonstrated 

 that the transmitted light is polarised in a direction opposite 

 to that of the polarisation of the reflected rays ; the one 

 being polarised in the plane of incidence, the other perpen- 

 dicularly to this plane. Mr. Arago has found, by some inge- 

 genious experiments which afi*orded him a very correct test, 

 that the quantity of light polarised by reflection, at the sur- 



