Optical distance measure converter by Emil Sydow
This rarely seen optical measuring instrument is signed Emil Sydow Berlin NW. Sydow’s optical company was the maker of the Helmholtz ophthalmoscope in 1852 that was used by the famous ophthalmologist, Albrecht von Graefe.
This unique device is from 1925. This optical distance measure converter is made of aluminum. The cleverly designed instrument has a single post about which both blades are able to rotate. There is a ball at the end of each blade.
The ball at one end of the instrument is meant to be placed at the rim of the patients brow. The ball at the other end is meant to be placed on the reading material that is to be read. On one blade is this distance in centimeters. At the intersection of the two blades can be read this value in diopters of power. If for example the distance was 25 centimeters (~10 inches) then the corresponding dioptric reading would be 4.0 That means that a four diopter plus lens would be needed to focus the image for this person holding material at 10 inches.
This is very close for reading, so there is a feeling that this may have been used for people with a vision handicap who needed to hold things more closely for reading than the average person over 40 who needs an ordinary pair of reading glasses.