My Dad used to do a 'trick' where he would zero out a 6 decimal place readout on the big ground glass comparator.. You could then move the table anywhere you w

My Dad used to do a 'trick' where he would zero out a 6 decimal place readout on the big ground glass comparator.. You could then move the table anywhere you wanted in X/Y, cover the display, and he would bring it back to 0.0000X by eye :-) He actually went to some institute in Sweden that did ultra-accurate measurements to show them how he did it. I think the 'trick' was, he moved the shadow till it was just past the blueprint, and back it off till a sliver of light shone thru, rather then trying to 'walk' the shadow up to the blueprint.. Neat stuff - but I decided on a tech career when I was mucking out the Holyroyd mill and he turned the cutting pumps on :-P Being drenched in cutting oil convinced me I liked computers better :-P