382 Mr. Brougham’s Experiments and Observations 
other, like all refractions, begins at some small distance before 
the rays enter. 
The writers just now quoted assert, that if the crystal be 
turned round so as to assume different positions, there is one 
in which the line appears single. The fact is very different, as 
follows. When the crystal is turned round, the unusual image 
moves round also, and appears above the other; the greatest 
distance between the two images is when they are parallel to 
the line bisecting one of the acute angles of the parallelogram 
through which the rays pass ; when the images are parallel to a 
line bisecting one of the obtuse angles they seem to coincide ; 
but they will be found, if observed more nearly, to coincide only 
in part. Thus (in fig. 9. ) A B and C D are the two black lines 
at their greater distance, and their extremities A and C, B and D 
are even with one another ; that is, the figure formed by join- 
ing A and C, B and D is a rectangle. But in the other case 
(fig. 8.) A B and C D being the lines, the space C B (equal in 
depth of colour to the real line on the paper), is the only place 
in which the lines (or images) coincide. The space AC of AB, 
and B D of C D are still of a light colour, and the two lines AB 
and C D do not coincide, by the difference AC or BD; that is, 
by the difference O P, the greatest distance (fig. 9.). In short, 
the unusual line’s extremities describe circles (in the motion of 
the crystal) whose centres are the extremities of the usual line, 
and whose radii are the greatest distance. From this it appears 
evident, that the unusual image is formed within the crystal, 
and turns round with the side of the particle, or rhomboidal 
mass of particles, which forms it. Farther, it is evident that 
the power which produces the division of the incident light, is 
very different from common refraction, from the motion, and 
