The precision of alignment attainable with a single slit, a double slit and cross wires, placed respectively at the three points to be aligned, can be about at least 0.17 in., provided use is made of pointings on transition colors in the interference pattern. Practical uses of this method are described, including technical measurements and laboratory experiments of very high precision. Among the last is a spherometer for large radii, that is also an absolute flatness tester with a precision of about λ/100. The method can be expanded to other types of measurement, when instead of a double slit a grating of coarse structure is substituted. Alignment in two directions simultaneously can also be achieved by a modification of the diffracting screen.
A. C. S. VAN HEEL, "High Precision Measurements with Simple Equipment," J. Opt. Soc. Am. 40, 809-814 (1950)