Aung. 11, 1870] 
NATURE 
299 
TRANSMISSION OF POLARISED LIGHT 
THROUGH UNIAXAL CRYSTALS 
po appearances presented by the transmission of 
polarised light through crystals, have long been 
known as the most magnificent in Optics. It is our 
intention in this paper to give an account of the more 
recent observations which have been made respecting the 
phenomena exhibited by uniaxal crystals, accompanied | 
by such an explanation as will, we hope, render them 
intelligible to persons very slightly acquainted with science. 
We shall therefore avoid as much as we can the use of 
technical terms, and assume as little as possible to be 
previously known to the reader. 
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Fic. 1. 
Common light consists of undulations of the supposed 
zetherial medium, in which the vibrations of each particle 
are perpendicular to the direction of the wave’s motion, 
and in every conceivable direction which this condition 
adimits. 
Suppose now a ray of sunlight reflected from a plate of 
unsilvered glass. The principle of the composition and re- 
solution of forces plainly enables us to regard it as made 
up of two sets of vibrations, one-in the plane of reflection, 
and the other perpendicular to it. Now for reasons 
which cannot be explained to a reader unacquainted 
with mathematics,* it is found that the vibrations of the 
reflected ray in the plane of reflection, when the angle of 
Fic, 2. 
reflection is equal to 56° 30’, wholly disappear, so that the 
reflected ray consists entirely of vibrations perpendicular 
to the plane of reflection, and is said to be polarised. 
The reader will carefully remember the distinction between 
common light and polarised light. Common light is light 
in which the vibrations occur in every conceivable plane 
passing through the ray; polarised light is light in which 
the vibrations occur in only one plane of fixed inclination 
passing through the ray. 
Now let a ray of sunlight fall upon a plate of unsilvered 
glass at an angle of about 56°, let the reflected ray be 
received by another glass plate at the same angle, and be 
* The mathematical reader is referred to Green’s ‘‘ Memoir on the Re- 
flection of Light,” Cambridge Piilosophical Transactions, vol. vii. 
thence reflected to the eye. Suppose the position of the 
mirror to be such that the planes of first and second inci- 
dence and reflection are perpendicular to each other. 
Then remembering what we have just stated, we see that 
the vibrations of the ray reflected from the first mirror lie 
altogether in the plane of second incidence. These vibra- 
tions, therefore, will all be destroyed by the second 
reflection, therefore nothing but a dark spot will be per- 
ceptible to the eye. 
If now the position of the second mirror be shifted, so 
that the planes of first and second incidence coincide with 
each other, while the angle of second incidence remains 
unaltered, the vibrations of the incident ray will all lie 
perpendicular to the plane of second incidence, and there- 
fore a bright spot will be visible to the eye. 
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Fic: 3: 
Let now the second mirror be gradually turned round 
an axis situated in the direction of the ray incident upon 
it from the first position into the second, then the angle of 
second incidence will remain unaltered during the revolu- 
tion, while the plane of second incidence will revolve with 
the mirror, and, as might be expected, light will begin to 
be visible to the eye and continually increase, till it attains 
its maximum in the second position of the mirror. It is 
usual to call the first mirror the polariser, the second the 
analyser. 
Let a plate cut from a transparent crystal be interposed 
between the polariser and analyser thus arranged, luminous 
curves of varying colours intersected by dark bands will 
be visible to the eye. Before describing these gorgeous 
appearances, we must say a few words on the modifi- 
cation light undergoes in passing through a crystal. 
When a ray of light passes through a crystal, it is in 
general divided into two, so that if a plate of crystal 
be interposed between the eye and a luminous point, the 
point will appear doubled. But in all crystals there is one, 
and in some two fixed directions in which no bifurcation 
of the ray takes place. If we look along one of these 
lines, called the optic axes, we shall see only one image 
of the object of vision. Crystals with one axis are called 
uniaxal crystals ; those with two axes are called biaxal 
