76 THE HAMILTON ASSOCIATION. 
Now Huygens had observed that the two beams of light into 
which a single incident beam traversing Iceland spar is broken had 
peculiar properties, which Newton described as ‘‘two-sides.” In 
1810 Malus, when examining the light of the sun reflected from the 
windows of the Luxenbourg Palace in Paris, observed that the 
reflected light had properties similar to those possessed by lights 
which has traversed Iceland spar. It was he who applied the term 
“polarised ” to describe the effect. About the same time it was 
discovered that light which had passed through tourmaline in the 
proper direction was also polarised. Thus the problem of deter- 
mining the precise nature of polarised light became a live question. 
It was solved by Fresnel and Arago, who made many beautiful 
experiments on the interference of polarised light. The conclusion 
reached by them was that the vibrations in light cannot be longi- 
tudinal as in sound, but must be transversal, 7. e., in a direction 
perpendicular to the direction of propagation ; and further, that in 
polarised light this transverse motion must be limited to a certain 
direction. Thus, if the axle of a carriage represent the direction in 
which the light is moving, then the particles of the ether may vibrate 
along any spoke if the light is common, but if it is polarised they 
are restricted to be along some particular spoke—for instance, in 
the vertical or horizontal plane. Thus polarised light is of a more 
simple, regular, restricted nature than common light. ‘This latter 
has a certain go-as-you-please freedom about it; the former is tied 
down to definite limitations. Hence when we use polarised light in 
an investigation we use a tool much simpler in nuture, and which 
will show more easily interpreted effects. 
Polarised light can be utilised in many important investigations 
and to show some transcendently beautiful phenomena. In working 
with it the common light is first polarised, usually by a Nicol prism— 
which is made from a crystal of Iceland spar and is named after its 
inventor—and then after passing through or being reflected from 
whatever we are investigating, it passes through a second Nicol. 
The first one is called the polariser, the second one the analyser, 
and the combination of the two with any suitable lenses is called a 
polariscope. Of course, in place of the Nicols we may use any other 
polariser or analyser, ¢. g., a slice of tourmaline or a glass plate 
inclined at the proper angle. 
