“THE ROSEN RULE” FOR OPTICIANS, English, c. early 20th century, signed “London Optical Company,” the 7-1/2″ (19 cm) long two-sided aluminum rule with a complex array of scales and devices for quantitative fitting of eyeglasses. Nineteen sliding, clampable rods conform to the shape of the nose, and adjustable targets align with the centers of the pupils; a rotating index provides degree conversions, plus there are quantized arcs, linear scales, etc. This unusual rule is in very fine condition. It is sufficiently rare that we have found no other trace of one.
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David and Yola Coffeen both have enjoyed academic careers, as planetary astronomer and as linguist/educator. But since 1982 (yes, 1982!) they have been full-time dealers in early scientific and medical instruments, under the name Tesseract. Selling primarily by catalogue (over 100 issued so far) they also have a web presence at www.etesseract.com, and can be contacted at email@example.com.