The many modifications of Helmholtz's eye mirror in the course of fifty years show that there was at first a tendency to simplification, to make the original instrument handier and more expeditious. Thus the small Liebreich's ophthalmoscope, a concave mirror with a clip behind the sight-hole for a few lenses, mainly destined to correct the physician's myopia, readily became very popular on account of its easy handling and the brilliancy of the fundus image, which never has been surpassed. The inconvenient clip was soon replaced by a disc revolving on the back surface of the mirror, putting a number of correcting lenses successively behind the sight-hole. This has been the most important practical improvement of Helmholtz's instrument. Large demonstration ophthalmoscopes soon followed, from Ruete's instrument to Thorner's, the latter being a marvel in distinctness and brilliancy of the picture, which is free from reflexes and larger than the erect image.
KNAPP H. A SIMPLE SINGLE-DISC EYE MIRROR.THE WORKING OPHTHALMOSCOPE FOR THE EYE SFECIALIST AND GENERAL PRACTITIONER, AND HOW TO USE IT.. JAMA. 1902;XXXIX(22):1363–1364. doi:10.1001/jama.1902.52480480005001a
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