Asttonomical and Nautical Collections. 173 



the same with respect to different directions : as they would if 

 the particles of the medium oscillated backwards and for- 

 wards in lines perpendicular to the directions of the rays. 

 But it is better to abandon all theoretical ideas of this kind 

 until we have entered more fully into the phenomena. 



It is not merely by passing through a crystal, which di- 

 vides it into two distinct pencils, that light receives this 

 remarkable modification ; it may also be polarised by 

 simple reflection at the surface of a transparent body, as 

 Malus first discovered. If we throw on a plate of glass a 

 pencil of direct light, inclined to the surface in an angle of 

 about 35°, and then place a rhomboid of calcarious spar in the 

 way of the reflected ray ; we remark, that the two pencils 

 into which it is divided by the crystal, are only of equal in- 

 tensity, when the principal section of the rhomboid makes an 

 angle of 45° with the plane of reflection, and that, in all 

 other cases, the intensities of the two images are unequal : 

 this inequality is the more sensible, as the principal section is 

 further removed from the angle of 45°, and finally, when it 

 coincides with the plane of incidence, or is perpendicular to 

 it, one of the two images disappears : the extraordinary image 

 in the former case, and the ordinary in the latter. Thus we 

 see that the light reflected by glass, at an inclination of 35°, 

 is similarly affected with the ordinary pencil, transmitted by 

 a rhomboid with its principal section in the direction of the 

 plane of reflection. The reflected pencil is said to be pola- 

 rised in the plane of reflection ; and in the same manner the 

 ordinary pencil transmitted by a rhomboid is said to be po- 

 larised in the plane of the principal section of the crystal ; 

 and we are obliged to say, on the other hand, that the extra- 

 ordinary pencil is polarised perpendicularly to the principal 

 section, because it exhibits in that direction the same proper- 

 ties which the ordinary pencil possesses in the plane of the 

 section. 



The surAice of water completely polarises light by reflec- 

 tion at the angle of 37° ; and at the surface of other trans- 

 parent bodies, in general, when the incidence is such that the 

 reflected may be perpendicular to the refracted ray. For the 

 discovery of this remarkable law, we are indebted to Dr. 



