5296 Discoveries of Erasmus Bartholinus. 
Bartholinas next supposes, that there are some directions in 
which the rays pass through the crystal unrefracted; and 
though in ordinary diaphanous bodies these directions are per- 
pendicular to their surfaces, ^yet in other bodies they may have 
another position. He likewise supposes, that half of the inci- 
dent pencil is refracted usually^ and the other half unusually ; 
or, what is the same thing, that the usual and unusual refrac- 
tions have the same power to refract the incident light. 
From this account of Bartholinus‘’s experiments, he appears 
to have discovered three important facts. 
1. That Iceland spar has the property of double refraction. 
That one of these refractions is performed according to a 
law which is common to all transparent solids and fluids, 
while the other is performed according to an extraordinary 
law which had not previously been observed by philoso- 
phers; and, 
3. That the incident light is equally divided between the 
ordinary and extraordinary pencils. 
Bartholinus does not seem to have transmitted the ordinary 
and extraordinary pencils through a second piece of Iceland 
spar, and it was therefore reserved for the celebrated Huygens 
to discover, by means of this experiment, the remarkable pro- 
perties which arise from the polarisation of these two pencils. 
Edinburgh, July 1. 1819. 
Art. XII . — Sketch of the Distribution of Rocks in Shetland. 
By Samuel Hibbert, M, D. M.W.S. Communicated by 
the Author. 
Of the group of islands bearing the name of Shetland, no- 
thing entitled to the appellation of a Chart, intended to compre- 
hend the whole of them, has yet been produced From in- 
formation on which I am most disposed to rely, conjoined with 
my own observations, the clustre, exclusive of Foula, the Voe- 
Skerries, and Fair Isle, distantly separated from the rest, may 
* I know of no actual surveys which have been made deserving confidencejt 
except of four or five miles of coast in the vicinity of Valey Sound, which was 
all that Captain Freston accomplished, in a chart, to which his name is affixed* 
pretending to delineate the whole of Shetland ; to this may be added a sketch of 
