Astronomical and Nautical Collections. 175 



face of a transparent body, is always equal to that which is 

 polarised by refraction. The enunciation of this remarkable 

 principle may be made still more general, if we say that 

 whenever light is divided into two pencils, without any 

 absorption, the same quantity of light, that is polarised in 

 the one, is found to be polarised in a perpendicular direc- 

 tion in the other. 



Having now studied the principal means of polarisation, 

 we are next to apply ourselves to the singular phenomenon 

 presented by polarised light, when it is thrown on the sur- 

 face of transparent bodies ; and it is to Malus also that these 

 important discoveries are due. We have seen that the light 

 reflected by glass at an angle of 35° was completely polar- 

 ised : this property is universal, and independent of any 

 anterior modifications of the light ; and, in fact, light which 

 has been polarised in any other manner is always found, like 

 common light, after the reflection, completely polarised in 

 the plane of incidence. Now w^e have remarked that a po- 

 larised pencil exhibited but one image in passing through a 

 rhomboid of calcarious spar, the principal section of which 

 was either parallel or perpendicular to its plane of polarisa- 

 tion ; that is, the ordinary image in the former case, and the' 

 extraordinary one in the other ; or the image of which the 

 plane of polarisation coincides with the principal section : 

 hence a pencil polarised in one plane cannot furnish, by any 

 immediate subdivision, an image polarised in a plane perpendi- 

 cular to it: and, generalising this principle, we must conclude 

 that a polarised pencil, thrown on glass at an inclination of 

 35°, with a plane of incidence perpendicular to its plane of 

 polarisation, is also incapable of furnishing any light polar- 

 ised in the plane of incidence, since this is perpendicular to 

 its own plane of polarisation : but the rays reflected at an 

 inclination of 35° are always polarised in the plane of inci- 

 dence ; consequently the incident pencil, which is polarised 

 in a direction perpendicular to this plane, can afford no 

 reflection. This conclusion was justified by the important 

 experiments of Malus ; and in the case which we are con- 

 sidering, there is no reflected light, the whole being trans- 

 mitted. But if, without changing the inclination of the 



