212 PHILOSOPHICAL TRANSACTIONS. [ANNO 1707- 



crystal on the particles of light in their passage through the body ; and from obs. 

 4 it is further evident, that it is not owing to the different properties which Sir 

 Isaac Newton conjectures the different sides of rays to have ; for if this were the 



cau^e, when the rays pass between 2 pieces of crystal or. "- *...».*. ■•~ ui " 



mmmma ^mci Mmmafi anoiner tact, mis-stated by Bartolin * and Rome de Lisle -j~, 



shows, that the unusual refraction takes place within the body, while the 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 

 2 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. 13, ab 

 and cd are the 2 black lines at their greater distance, and their extremities a and 

 c, b and d are even with each other ; that is, the figure formed by joining a and 

 c, b and d, is a rectangle. But in the other case, fig. 12, ab and cd being the 

 lines, the space cb, 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 bd of 

 cd are still of a light colour, and the 2 lines ab and cd do not coincide, by the 

 difference ac or bd ; that is, by the difference op, the greatest distance; fig. 13. 

 In short, the unusual line's extremities describe circles, in the motion of the crys- 

 tal, 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. Further, it is evident that the power which 

 produces the division of the incident light, is very different from common refrac- 

 tion, from the motion, and the effect taking place when the rays are perpendicular. 

 Suspecting therefore, that it might be owing to flexion, I made the following expe- 

 riment, which undeceived me. 



Observ. 5. I covered one side of a specimen of Iceland crystal, 3 inches deep, 

 with black paper, all but a small space T V of an mcn m diameter, and placed a 

 screen with a hole of the same size, 6* feet from the hole in the window-shutter of 

 my darkened chamber, so that the rays might pass through the screen, and fall on 

 a prism placed behind, to refract them into a small and well defined spectrum, 

 which was received on a chart 2 feet from the prism. This spectrum I viewed 

 through the crystal, and of course saw it doubled ; but the 2 images were by no 

 means parallel ; the unusual one inclined to the red, and its violet was considerably 

 farther removed from the violet of the other, than the 2 reds were from each 

 other : which shows that the most refrangible or least flexible rays were farthest 

 • Experiment Cryitalli. — Orig. ^ Cristallographie, vol. i. — Orig, 



