THE CHEMISTRY OF THE HIVE. 
583 
When the light is polarised, all its waves are in one 
plane. To bring this before the eye, let us suppose 
the bent wire ab, Fig. 125, to represent a simple 
wave, the plane of which is perpendicular — i.e.^ the 
oscillations are up and down ; then the wave, cd^ 
making its oscillations sideways, would be said to 
move in a horizontal plane. Any number of such 
wires, put at random side by side, would represent 
an ordinary beam, the vibrations of which are in every 
plane, but if the wires all had their planes parallel, 
Fig. 125.— Diagram explaining Sugar Analysis by Polarised Light. 
ah, Ray with Perpendicular Plane of Vibration ; cd, Ray with Horizontal Plane ; 
e/, Ray with its Plane undergoing Rotation ; t. Tube containing Sugar to be 
analysed ; gr 1 and gr 2, Gratings. 
they would represent a beam of polarised light. Let 
gr I be a grating of perpendicular wires, set wide 
enough to pass ab or cd, if straight ; then it is clear 
that ab, although bent, would still pass, while cd 
would be stopped. Many crystals arranged as prisms 
are optically such gratings ; they will only allow rays 
to pass in one plane. Common light in going through 
them is therefore polarised. If a second grating, gr 2^ 
be placed behind the first, with its wires horizontal, ab 
