transmitted through crystallized Bodies. 3S\ 



4. Oriental ruhy. This doubly refracting crystal afford* 

 beautiful rings, in which, owing to the colour of the mineral, the 

 predominant colours are crimson, light blue, and blueish-green. 

 Tiie central spots were distinctly visible ; and though the crystal 

 was -r-'j»g- of an inch thick, the rings appeared to be larger than 

 those given by tojjaz -pni/o- ^f an inch thick. 



5. Emerald. The coloured rings formed by this stone are 

 principallv blue and greenish yellow, the least refrangible rays 

 being exthiguished by the green colour of the mineral. 



C. Native hydrate of magnesia. This mineral affords very 

 distinct segments of coloured rings when the light is transmitted 

 in a direction nearly perpendicular to the surface of the laminae. 

 Owing to the imperfect structure of the plates, I could not ob- 

 tain a measure of the diameter of the rings. 



7. Amber. As this substance possesses no crystalline form, 

 and does not split into laminae, 1 found it impossible to make 

 any satisfactory experiments with it. The enormous breadth of 

 its coloured rings was conspicuous in every specimen ; but though 

 I ground and polished more than twenty plates of it, 1 could 

 not obtain one which exhibited any thing more than broad-co- 

 loured segments. With a parallelopiped of amber 0*566 of an 

 inch long, 0*300 broad, and 0*367 deep, the coloured segmentt 

 were visible in every direction in which the light was trans- 

 mitted. They appeared most distinct through the thickness 

 0*367 ; and through the thickness 0*566 they were still so 

 broad, that no more than one colour of each ring could be 

 seen. In a piece of amber -f-^ of an inch thick, the ring', were 

 broader than in a plate of topaz ^\ of an inch thick. 



8. Ice. The difficulty of making experiments upon ice with- 

 out melting it, the want of a crystalline form, and the imprac- 

 ticability of shaping into parallel plates, prevented me from 

 obtaining any accurate results. The following experiments, 

 however, will throw some light upon this subject. 



A piece of ice -'g- of an inch thick gave rings much broader 

 than those exhibited by a plate of topaz ^l. of an inch thick. 

 The rings were also seen by the reflection of common light from 

 the posterior surface of the ice, the light reflected from the an- 

 terior surface being extinguished by a ]?rism of calcareous spar. 



A piece of ice ^ of an inch thick exhibited rings larger than 

 those given l)y a plate of topaz -^ of an inch thick. The 

 breadth of one of the fringes shown by a plate of ice -f^^ of an 

 inch thick was nearly 5" 26', which compared with the results 

 already mentioned, gives -' J\f for the thickness of a plate of to- 

 paz that would produce a fringe of the same magnitude. Hence 

 the thicknesses of ice and topaz that give rings of equal size are 

 ** ■3'V*- to -^'-ijOT as 8*95 to 1, which i« nearly the inverse ratio 



