07' the Ahsoi'ptioii of Cominon Light. S47 
through it at common light, exhibits no other colour but yel- 
low, mixed with a small quantity of blue, polarised in an oppo- 
site plane. The ordinary image at c and d is yellowish-brown, 
and the extraordinary image faint blue, the former acquiring 
some blue fays, and the latter some yellow ones from c and d to 
a and 5, where the difference of colour is still highly marked. 
From a and b towards p and p' the yellow image becomes faint- 
er, till it changes into blue, and the weak blue image is rein- 
forced by other blue rays, till the intensity of the two blue 
images is nearly equal. The faint blue image increases in in- 
tensity as the incident ray*approaches from c and d to p andj5', 
and the yellow one acquiring an accession of blue light, becomes 
bluish-white. From p and p' to o, the ordinary image is whitish, 
and the other deep blue, but the whiteness gradually diminishes 
towards o, where they are both almost equally blue, the ordinary 
i image being more luminous at o. From a and h to o, the yellowish 
image becomes more blue, and the bluish image also more blue. 
The principal axis of lolite is negative, and its greatest 
refracted image is purplish blue, while the ordinary or least re- 
fracted image is yellowish-brown, passing into one another as 
above described. The index of the ordinary refraction is about 
1.549 ; and the mineral belongs to the prismatic system of Mohs, 
though both [this mineralogist and M. Haiiy place it under the 
rhomboidal system. 
These properties of lolite are finely seen in two excellent 
specimens in Mr Allan’s cabinet, which I directed to be cut in 
the manner represented in Fig. 9. 
Art. VI.— Ow the Thermometer., as an mdicator of a Ship'^s 
approach to Land or Soundings^ with extracts from a Ther- 
mometric Journal hept on hoard the ship Asia of Scarbo- 
rough, on a Voyage from New Orleans to Gibraltar, in 
August, September and October 1818. By Mr Andrew 
Livingston. 
It is now placed beyond dispute, as a matter of fact, that 
the thermometer indicates the proximity of the shores of 
the middle part of the coasts of the United States of America; 
but I am not aware whether any experiments have been made 
