Imperial Institute of France. 807 
municating its power of developing the prismatic colours to 
resin, wax, cement, gold-leaf, lead, &c. and this communicated 
quality is retained even after exposing the body to a considerable 
degree of heat, so that it is not actually fused. This fact was 
discovered by Dr. B. in consequence of having cemented a piece 
-of polished mother of pearl on a goniometer to take the angles 
of refraction. He cut and polished the mother of pearl as well 
as possible in various modes, but still found it retain its strice or 
cavities resembling the fine lines used by map engravers to define 
the limits of land or water. To these striae he attributed in some 
degree the formation of the prismatic colours. 
IMPERIAL INSTITUTE OF FRANCE, FOR THE YEAR 1813. 
[Continued from p. 232.] 
Mathematical and Physical Sciences. 
The parabola itself is only a limit, a mere single case be- 
tween the ellipse and hyperbola, the dimensions of which may 
vary ad infinitum. Hence there is scarcely any probability of 
a parabolic orbit. The hyperbolas are scarcely perceptible, 
and from experience it would seem that the ellipses are hardly 
more so. Hyperbolic comets never return—elliptical return 
after a long interval. There is no reason for being surprised 
that only one has hitherto returned constantly. 
Laplace has discovered in the attraction of Jupiter a probable 
cause why the comet of 1770 has not appeared eight times since 
that period. Perhaps it is entirely evaporated, or reduced to 
a nucleus so small, and so little luminous, that it will remain 
for ever invisible. Both these explanations are not incompa- 
tible,; and they may have concurred in producing an effect 
which has excited the curiosity of mathematicians and astro- 
homers. i 
It may be presumed that this new hypothesis will consider- 
ably diminish the importance ‘of comets: it is similar to the 
opinion of the ancients, who considered comets as temporary 
collections of vapour which are speedily dissipated and destroyed. 
Hence the astronomers of old did not think them worthy of 
being scrupulously observed. All the difference is, that Aris- 
totle places comets below the moon; whereas, according to Dr. 
Herschel, they are formed beyond the planetary system, and 
last much longer, since it is only in their perihelion that they 
are liable to lose a part of their substance. But if they become 
invisible, it is the same thing to us as if they did not. exist. 
M. Burckhardt’s Memoir on the Quantity of Matter in the 
Planets. 
The formulas of MM. Lagrange and Laplace enable us to 
U 2 assign 
