Double Refraction and Polarisation of Light. S91 
law of refraction discovered by Snellius, it is called the ordi- 
nary pencil or the ordinary image., while the other pencil or image 
is called the extraordinary one, from its being refracted according 
to a law different from the ordinary law. 
The light which composes the two images or pencils thus 
formed by double refraction, possesses properties different from 
all ordinary light, and as these properties are related to the 
different sides of the rays or pencils, the rays or pencils of 
light which possess such properties, are said to be polarised^ 
or to indicate polarity, in the same way as particles of iron in 
the vicinity of a magnet indicate polarity, by possessing a pro- 
perty in one of their sides or extremities, which they do not possess 
in their other extremity. Now, as the diameter of a beam of light 
may be reduced to a very small magnitude, and as every por- 
tion of the beam in the direction of its length has a progressive 
motion, and possesses the same properties as the whole beam, 
we may call tfiese minute portions Particles of Light and speak 
of the Sides or Poles of these particles, without considering whe- 
ther light is composed of material particles issuing from the 
^uffs body, or is merely the undulation of an elastic medium. 
Period I. Account of the Discoveries of Erasmus Rar-^ 
tholinus, respecting Double Rfraction. 
About the middle of the seventeenth century. Dr Erasmus 
Bartholinus, a physician at Copenhagen, and the author of seve- 
ral excellent works on geometry, procured from some of tx^e 
Danish merchants that frequented Iceland, ‘‘ a crystal stone like 
a; rhombick or rhomboid prism, which, when broken into small 
pieces, kept the same figure.” It was dug out of a very high 
mountain, not far from the Bay of Roerfiord, in 65° of latitude. 
Its whole body was rather clear than bright, of the colour of 
limpid water, but that colour, when it was immersed in water 
and dried again, became dull.” With this substance, which, 
from its locality, was called Iceland Crystal., Bartholinus made 
a number of experiments both chemical and optical; and having 
discovered some of the singular effects which it produced upon 
light, he published an account of them at Copenhagen in 1669, 
under the title oi Experinienta Cry stalli Islmdiei iDis-dictdastici., 
quibus mira et insolita ref radio detegitur. There does not ap- 
