98 Dr Brewster on a s^inffular Structure in the Diamond. 
iiated surfaces, I shall conclude with the following summa- 
^ J • 
1. While viewing a coloured surface occupying the whole 
sphere of vision, the sensation is less decreased than the 
sensibility is increased in darkness in a given time. 
52. While the retina is acted upon by a stronger and weaker 
light together, the sensation produced by the stronger light 
is greater, and the sensation produced by the weaker light 
less, than what is produced by these different degrees of 
light when they occupy singly the whole sphere of vision. 
3. After the retina is acted upon by a stronger and weaker 
light together, the sensibility is increased where the sensa- 
tion (2.) was diminished, and diminished where the sensa- 
tion was increased. 
4. In certain circumstances the retina, without the direct 
agency of light, is thrown into the same state which light 
produces. Experiments 4, 7. and 8, 
Art. XVII. — Notice respecting a Singular Structure in the 
Diamond. By DxWin Brewster, LL.D. F. R. S. Lond. 
and Sec. R, S. Edin 
Having been lately engaged in examining the optical 
structure of Amber I was surprised to observe that it exhi- 
bited phenomena almost exactly the same as those which I had 
long ago noticed in several flat Diamo7ids, which belonged to 
Mr Robison, a member of this Society. In order to compare 
the two structures with greater attention, I re-examined nine of 
Mr Robison’s diamonds. In all of them I remarked that the 
polarised tints were arranged in streaks, like those developed by 
amber ; but one of the diamonds exhibited a new phenomenon 
of a most unexpected kind, which is the only fact in the natural 
history of this body that promises to throw light upon its origin 
and mode of formation. 
* This notice was read before the Royal Society of Edinburgh, on the 3d 
April 1820. ' • 
♦j- See this Journal^ vol. ii. p. 332. 
