When light goes through a crystal
Of an anisotropic kind
It's split two ways, one ray goes fast
While the other's left behind.
And both of these are polarised
In completely different planes -
A sheet of polaroid gets rid of one
While the other one remains.
And so the two directions
Where the rays of light vibrate
Have different refractive indeces
For the different light travel rates.
So next we draw a small ellipse
With long and short axes
And these two perpendicular lines
Show the two allowed VDs.
But now here comes the cunning bit -
The shape is drawn whereby
The length of each line's proportional to
That one VD's R.I.
The optical indicatrix,
An ellipsoid is the norm
We find it buried deep inside
Our mineral crystal's form.
We know about the crystal,
It's symmetry's the key -
It tells us how the indicatrix
Will orientated be.
So now we cut the crystal up
As flat as is the floor;
The section through the indicatrix
Is the ellipse we had before.
So if it's long and thin we know
The birefringence is high
And the colours we see in the microscope
Will be pleasing to the eye.
But if it's rounder we will say
The colours we will lack
And if the ellipse is completely round
The crystal will be black.
And now we look at crystals real
And take a note beside
Of the deepest colour that we view
And the thickness of the slide.
The birefringence we calculate
If we take a little care
And from our trusty indicatrix,
We know what mineral's there.
And we can use this same technique
As often as we please
To identify lots of minerals
With total utter ease.
And that is why I love it so
If you only use it right -
The optical indicatrix
Can help you see the light.