Consists of a variable density annulus on a glass plate and second plate with index to which a scale has been added. 2 ½" diameter.

Notes:

Sanger-Shepherd name on second plate. Address on box: Sanger-Shepherd & Co. 5, 6 & 7 Gray's Inn Passage. Red Lion St. High Holborn. WC. (before 1910).

With:

Sanger-Shepherd red box.