Dr, Brewster on some Properties of Ught, 103 



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 reflectioriy and not one ray of 

 it penetrates the agate. 



If a ray of light incident upon one plate of agate is received 

 after transmission upon another plate of the same substance, 

 having its laminae parallel to those of the former, the light 

 will find an easy passage through the second plate ; but if the 

 second plate has its lamin* perpendicular to those of the first, 

 the light will be wholly reflected, and the luminous object will 

 cease to be visible. 



Owing probably to a cause which will afterwards be noticed, 

 there is a faint nebulous light unconnected with the image, 

 though always accompanying it, and lying in a direction pa- 

 rallel to the laminag. This light never vanishes along with the 

 images, though it is evidently affected by the different changes 

 which they undergo ; and in one of the specimens of agate, it 

 is distinctly incurvated, having the same radius of curvature 

 with the adjacent laminse. This character of the nebulous 

 light I consider as an important fact, which may be the means 

 of conducting us to a satisfactory theory, and I am at present 

 engaged in examining it with particular care. 



This remarkable property of the agate I have found also in 

 the kindred substances of cornelian and chalcedony, and it is 

 exhibited in its full effect even when these bodies are formed 

 into prisms, and when the incident rays fall with any angle 

 of obliquity. In one specimen of agate, which has no veins 

 to indicate the direction in which it was cut, the images did 

 not vanish as before ; and in another specimen of the same 



