1830 .] 
On the Polarization of Light, 
10 7 
pencil may be supposed to be made up of a multitude of atoms, whose polar axes 
are in every possible direction, so that an equal number should be disposed to be 
diverted into either of the two planes, by the action of the forces inherent in the 
glass, of whatever nature these may be ; analogy leading always to assign to them 
a close connection with the forces producing reflection and refraction themselves, 
which are attributed to attractions and repulsions exercised by the atoms of any 
substance upon opposite poles, as it were, of the atoms of light. 
6- To render clearer the conception of the opposite states of polarization men- 
tioned above, each atom of light should be regarded as having the power of revol ving 
round its axis of motion, like a magnet upon its pivot ; then when its poles (C D, 
figure 10) are situated parallel to the plane of reflection in which the atom is mov- 
ing forward, it is said to be positively polarized : when they form a right angle 
therewith, the term negative polarization is employed- 
v, y • r . V? 
Jipjj 
* Ui ' 
foicLTUgcrflOn at-o.nyJ.nalt. 
iitd’. many plaLiJ 
7. U/gTit once transmitted at an angle of 35° 25', having all its axes of polarization 
fit to escape reflection, will not suffer further diminution : if a number of glasses 
he applied to the back of the first, a candle will look equally bright; and if the 
last glass be drawn back (as in figure II) it will be seen that there is no light 
reflected- 
8. When the number of plates is increased to 30 or 40, the transmitted light is 
found to be polarized at any angle of incidence, although feeble in brightness ; be- 
cause each succeeding glass tends to rob the ray of the atoms fittest for reflection, 
and to transmit the remainder. In practice, the glasses should not be too close, 
otherwise they may act as a single glass. 
9. Having now noticed the chief circumstances of the polarization of light by 
surfaces of glass, I may proceed to detail the peculiar modifications of the same 
action, developed by crystallized bodies. Most of these have the curious property of 
dividing a transmitted ray of light into two portions, so as to render two images 
visible. These are very distinct in rbomboidal, or as it is hence called, double-re- 
fracting spar; but it will be more convenient to speak first of the tourmaline, which 
itself furnishes a mode of analyzing the polarized light of other substances. 
10. If a thin prisinbe cut from the polished side of a tourmaline crystal, and a 
polarized ray be viewed through it, two images are seen towards the thinnest part of 
the prism. The ordinary image is not white like the other, but of a greenish yel- 
low colour, and it grows fainter as the tourmaline increases in thickness until it 
disappears total rf: a plate of tourmaline of this last thickness, therefore, will only 
