The College of Optometrists

This is posted on behalf of a mate!

When calibrating a Perkins Mark II, why do you have to take off the base (and you dont for mark I)

Thanks

To compensate for the weight of the tonometer head and prism holder, the tension on the springs in the Perkins adjusts for the 'attitude' of the tonometer.

Try:

1. Set the tonometer to '0'.

2. Remove the doubling prism.

3. Tilt the tonometer between horizontal and vertical.

You will notice that the prism holder moves backwards.

In the Perkins I, the tonometer head lies absolutely horizontal with the battery case on. The Perkins II requires the battery case to be removed to achieve this.

This answer is from observation only and is my first post to these boards - hope this helps!

AJ

I thought you lines the prism with a mirror with a scratch line on it. I didnot think you had to take the battery cover off on the mark two. Just line it up withthe mirror.

The prism holder should not move in any position of instrument - it should be balanced when set to 0 - it will lie in a loose floppy attitude when the instrument is level with the desk, and prism line up with the mirror scratch.