508 Curved Appearance of Comets’ Tails. (October, 
characteristics of this stone inan eminent degree. The small 
so-called emerald of the Hope colle¢tion, in the same Museum, 
is nothing more than a piece of green glass. The true 
emerald is not well adapted for the purposes of the cameo 
and intaglio cutter, and was seldom engraved in Europe, 
although some examples exist,—as the Duke of Buccleuch’s 
fine Medusa’s head on emerald (a Florentine work of the 
seventeenth century). In Persia, India, &c., on the other 
hand, engraved or at least inscribed emeralds are not un- 
common. But the transparent beryl and particularly the 
aquamarine have frequently been engraved with great skill 
and beautiful effect, both in classic and later times. Several 
good specimens of this kind are preserved in different public 
and private collections of gems, both in England and on the 
Continent. In the Bibliothéque Nationale of Paris there is 
a fine cameo of Julia, the daughter of Titus: the British 
Museum possesses some beautiful engravings on the same 
material, and there are others in the Museums of Florence, 
Naples, &c. 
Some perfectly transparent and flawless aquamarines 
have been found of enormous size. One Brazilian stone 
weighs 225 troy ounces, and is as big as the head of a calf! 
The writer of this notice has seen one in London, of a rough 
spherical form, of perfect purity, though pale in tint, and 
nearly 4 inches in diameter. Mr. Hope possesses, in the 
hilt of Murat’s sword, an aquamarine of remarkable size: 
there is also a fine richly-coloured stone amongst the Crown 
jewels in the Tower. In many private collections and in 
public galleries the different varieties of the beryl may be 
found in abundance: there is a good series of specimen 
crystals in the Mineralogical Department of the British 
Museum. 
V. ON THE CURVED APPEARANCE OF 
COMETS’ TAILS. 
By Lieut.-Col. Drayson, R.A., F.R.A.S. 
es 
Ae our paper on the Pole Star and Pointers in this Journal, 
i] we called attention to the fact that what would appear 
as a straight line to an observer in one part of the 
world, would appear as a curve or arch in the heavens to 
an observer on another part of the earth’s surface. 
As an illustration of this law, we called particular attention 
