The reticle pattern consists of a central rectangle, subdivided into 9 smaller rectangles with a number of circles and dots (globes) along the top and bottom edges. Each circle is marked with the diameter in units, 250 units being the length of the large rectangle. Rectangle size is nominally 4.5mm x 2.025mm and circle/dot sizes in microns are nominally 450, 360, 270, 225,180, 145, 110, 74, 37 and 18.

Reference: H.S.Patterson and W.Cawood.Transactions of the Faraday Society, Vol. 32 Feb 1936.“The Determination of Size Distribution in Smokes.” Pp. 1084-1088.

Glass Type and Thickness

Soda Lime 1.5mm thick

Diameter Tolerance

+0/-0.1mm

Orientation

Correct reading through glass