ISIS.] 



Scientific Intelligence, 



305 



particles escaped refraction. On the contrary, when the princi- 

 pal section of the crystal was perpendicular to the plane of 

 i-eflection, the extraordinary ray was partly refracted and re- 

 flected, while the ordinary ray was refracted entire. 



While Dr. Brewster was employed in repeating the experi- 

 ments of Mains, and observing the effect produced upon light 

 by transmitting it through transparent and imperfectly transpa- 

 rent bodies, he was struck by a singular appearance of colour in 

 a plate of agate. This plate, bounded by parallel faces, was 

 about the 15th of an inch in thickness^ and was cut in a plane 

 perpendicular to the laminas of which it was composed. This 

 agate was very transparent, and gave a distinct image of any 

 luminous object. On each side of this imagq was one highly 

 coloured, forming w^ith it an angle of about 10°, and so deeply 

 affected with the prismatic colours that no prism of agate, with 

 the largest refracting angle, could produce an equivalent disper- 

 sion. Both the coloured images and the colourless image were 

 found to be polarized. Dr. Brewster found that when the image 

 of a taper, reflected from water at an angle of 52° 45^, is viewed 

 through a plate of agate, having its laminae parallel to the plane 

 of reflection, it appears perfectly distinct ; but when the agate is 

 turned round, so that its laminae are perpendicular to the plane 

 of reflection, the light which forms the image of the taper 

 suffers total reflection^ and not one ray of it penetrates the 

 agate. 



He found likewise that if a ray of light incident upon a plate 

 of agate be received after transmission upon another plate qi the 

 same substance, having its laminae parallel to those of the for- 

 mer, the light will find an easy passage through the second 

 plate ; but if the second plate has its laminae perpendicular to 

 those of the first, the light will be ivholly reflected^ and the 

 luminous object will cease to he visible. 



But the most curious observation made by Dr. Brewster on 

 the agate is the presence of a faint nebulous light, unconnected 

 with the image, though always accompanying it, lying in a 

 direction parallel to the laminjB. This unformed light never 

 vanishes along with the images ; and in one of the specimens of 

 agate it is distinctly incurvated, having the same radius of cur- 

 vature with the adjacent laminse. Dr. Brewster found the same 

 property in the carnelian and chalcedony, minerals of which the 

 agate is usually composed. Dr. Brewster ingeniously conjectures 

 that the structure of agate is an approach to that particular kind 

 of crystallization which occasions double refraction, and that the 

 nebulous light is an imperfect image arising from that imperfec- 

 tion of structure. He conceives that the phenomena of double 

 refraction are produced by an alternation of laminjfi of two 

 separate refractive and dispersive powers. Thus in calcareous 



Vol. I. N° IV. U 



