148 Account of Huygens’ Theory (f' Double Refraction. 
one side by binges, on the other by hooks and eyes ; 
the centre one contains the comb, and the outer ones 
the glass, which is placed one«third of an inch from the 
inner edge of the frame, to afford a passage for the bees 
between the comb and the glass, a a are two lighting 
boards to the two entrances, either of which may be 
opened at pleasure. 
Fig. 5. is the stool, fixed upon a square block n, and made to 
turn upon a pivot 5, which is driven into the ground. 
The entrances, c, are cut, one in each side, in the thick* 
ness of the stool, sloping upwards to the floor. When 
the observer has viewed the bees on one side, and wishes 
to see the other also, instead of sitting in front where 
they are busy in coming out and going in, and thus ex- 
posing himself to their stings, he has only to shut the 
front entrance, wheel round the hive on its pivot, and 
open the other entrance, d d are two iron rods fixed on 
the stool, and which support the hive by two staples 
in the centre frame. 
Art. ^'K.Yll.^Historical Account of Discoveries respecting 
the Double Refraction and Polarisation cf Light, Continued 
from Vol, ii. p. 171. 
Period II. Containing an Account of Huygens'* Theory 
(f Double Refraction. 
Having succeeded in explaining, in a very satisfactory 
manner, the refraction of ordinary transparent bodies, by means 
of spherical emanations of light, Huygens was naturally led to 
suppose, that as Iceland spar had two different refractions, it 
must also have two different emanations of waves of light, one 
of which might be propagated in a spherical form in the ethe- 
real matter spread through the crystal. He conceives that this 
ethereal matter exists in greater quantity than the solid particles, 
and is alone capable of producing transparency. From these sphe- 
rical undulations, which are propagated more slowly within the 
crystal than without it, proceed the phenomena of the ordinary 
