Movie: How a Two-Circle (Theodolite) Reflecting Goniometer works

Presented by archivist Patricia Egan, scientist Ross Pogson demonstrates a Two-Circle (Theodolite) Reflecting Goniometer, an instrument for measuring the angles between crystal faces.

Subtitles:Chris Lang

© Australian Museum
Patricia Egan & Anna Namuren
Dr Ross Pogson & Patricia Egan


PE: Hi, I’m Trish and I work in the archives of the Australian Museum. And I’m here today with Ross who is our Collection Manager of the Minerals collection. And we’re here today to talk about one of our most significant scientific instruments, this goniometer.

RP: To use this instrument, a small crystal like this small quartz crystal has to be placed at the centre of the instrument. So this nut is undone and a spindle is pulled out. The spindle has a cup in the top where goniometer wax could be melted and attached to the top and the crystal attached to the wax. But these days we can use Blue-tack.

When the crystal is in place, a strong light is shone through the collimator which is a group of lenses which further concentrates the light into a fine beam to shine on the crystal face. Before you do that you have to rotate this diaphragm on the end to choose a aperture shape, like a diamond or a rose shape. These shapes are called targets.

And the light which goes through and hits the crystal will have a pattern to it and that's called a signal. So the aim is to bounce light off the faces of the crystal and into this telescope.

So it's a matter of moving this axis on the vertical wheel, in and out, until the crystal face you want is at the intersection of the horizontal and vertical axes of the instrument. I’m just moving the crystal axis out.

Then that is clamped into position. Then the telescope is rotated around the axis until you pick up the signal from the collimator. You look through the telescope until you see the signal superimposed on the crosshairs. You can look at the reflected signal or you can look at the actual surface of the crystal itself by flitting, up and down, this auxiliary lens.

And there is a magnifier on the end of the telescope. And you can rotate this axis and bring up each crystal face in turn to reflect the light.

So you take a reading from the vertical circle, you take a reading from the horizontal circle, and the two numbers are like a latitude and longitude for describing the position of the crystal face.

You can read the scale to only a couple of minutes of arc and there is a travelling microscope here for looking at the scale. And when you’ve made a list of all the angles of the faces you can reconstruct the whole crystal in a drawing projection on a piece of drawing paper.

And this was used in publications and the angles between the crystal faces were constant and were representative of that mineral.

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