I photographed the print with the micrometer in contact and on comparing this with a genuine thumb-print, also photographed with the same micrometer in contact, I found that the suspected print was larger by the fortieth of an inch, from one given point on the ridge-pattern to another given point.

I planned an engine for describing ellipses by the polar equation A/(1 + e cos theta) and tried to make a micrometer with silk threads converging to a point.

This was done by setting the eye-piece approximately to the expected deflection, and turning the whole micrometer about a vertical axis till the cross-hair bisected the circular field of light reflected from the revolving mirror.

The columns are sets of readings of the micrometer for the deflected image of slit.

I have a number of these micrometers of different scales, and I find them invaluable in examining cheques, doubtful signatures and such like.