102 Dt\ Brewster on some Properties of Light, 



of incidence at which this property was communicated to light 

 by reflection from different substances, I made a variety of 

 experiments, with the view of discovering if a similar character 

 could be impressed upon light by its transmission through 

 bodies, either wholly or imperfectly transparent. All these 

 experiments afforded no new result, and every hope of disco- 

 vering such a property was extinguished, when my attention 

 was directed to a singular appearance of colour in a thin plate 

 of agate. This plate, bounded by parallel faces, is about the 

 fifteenth of an inch thick, and is cut in a plane perpendicular 

 to the laminae of which it is composed. The agate is very 

 transparent, and gives a distinct image of any luminous object; 

 but on each side of this image is one highly coloured, forming 

 with it an angle of several degrees, and so deeply affected 

 with colour that no prism of agate, \^ ith the largest refracting 

 angle, could produce an equivalent dispersion] Upon exa- 

 mining this coloured image with a prism of Iceland spar, I 

 was astonished to find that it had acquired the same property 

 as if it had been transmitted through a doubly refracting 

 crystal, and upon turning the Iceland spar about its axis, the 

 images alternately vanished at every quarter of a revolution. 

 My attention was now directed to the common colourless 

 image formed by pencils transmitted perpendicularly through 

 the agate ; and by viewing it through a prism of Iceland spar, 

 it exhibited all the characters of one of the pencils produced 

 by double refraction, the images alternately vanishing in every 

 quadrant of their circular motion. 



When the image of a taper reflected from water at an angle 

 of 52° 45', so as to acquire the property discovered by Malus, . 

 is viewed through the plate of agate, so as to have its laminae 



