Help with HNC assignment

Whilst searching for information on the web I came across your list.
I am struggling to find information on a number of topics for an
assignment for my HNC. If anyone can help with links or information
I would be most grateful. The topics are:

Abbes principles
Magnescales
Optical gratings
Encoders
And general info of control of machine tools.

Many thanks in anticipation,

Pat Ford.