On some Properties of Light. 287 
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 co- 
lour that no prism of agate, with the largest refracting 
angle, could produce an equivalent dispersion. Upon ex- 
amining this coloured image with a prism of Iceland spar, 
I was astonished to find that it had acquired the same pro- 
pertv as if it had been transmitted through a doubly re- 
fracting 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 com- 
mon colourless image formed by pencils transmitted per- 
pendicularly 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 lamine parallel to the plane of reflection, it ap- 
pears perfectly distinct; but when the agate is turned 
round, so that its lamine 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. 
If a ray of light incident upon one plate of agate is re- 
ceived after transmission upon another plate of the same 
substance, having its laminz parallel to those of the former, 
the light will find an easy passage through the second plate; 
but if the second plate has its laminz perpendicular to those 
of the first, the light will be wholly reflected, and the lu- 
minous object will cease to be visible. 
Owing probably to a cause which will afterwards be no- 
ticed, there is a faint nebulous light unconnected with the 
image, though always accompanying it, and lying in a di- 
rection parallel to the lamine. 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 laminz. 
This character of the nebulous light I consider as an im~ 
portant fact, which may be the means of conducting us to 
a satisfactory theory, and I am at present engaged in exa- 
mining it with particular care. 
This remarkable property of the agate I have found also 
in 
