This test slide is made in the UK under licence from the National Physical Laboratory. 

It is an epoxy replica of a master slide produced and certified by that laboratory. 

The replicas are mounted on microscope slides of 1.2 mm thickness with cover glass of 0.17 mm thickness. The purpose of the slide is to provide a standard means to check the performance of phase microscopes prior to the analysis of asbestos. The pattern consists of seven bands of twenty lines with widths ranging from 0.25µ to 1.1µm. 

A satisfactory system will detect block 5. Full details are supplied with the slide.