Extremely rare.
In 1851 Helmholtz published an article on his ophthalmoscope. The instrument, made mainly of brass, was constructed by Rekoss,instrument-maker to the University of Konigsberg. It consists of a lens-holder with a viewing hole of 3 mm diameter, a frame containing three glass plates on top of each other, and seven negative correcting lenses.
The glass plates are attached at an angle of 56 degrees, the polarization angle, to the instrument and form a semi-transparent mirror.
The examiner lets light from a light source fall on the glass plates at an angle of 56 degrees and directs the reflected light bundle into the subject’s eye, which is directly opposite his eye. The examiner can look through the sight hole with the beam of light into the subject’s eye.
The above photo was kindly allowed to be taken by Gary Edwards, M.D., by the curator of the Berliner Medizinhistorisches Museum der Charité in May 2010. The museum’s specimen is one of only a handful remaining in the world.
Museum, Ophthalmoscopes
