Von Helmholtz's ophthalmometer: historical review and experience with one of the last surviving original devices.

Hermann von Helmholtz (1821-1894) was one of the most important scientists of the nineteenth century in optics and ophthalmology. One of his significant contributions in the field of vision sciences was the invention of the ophthalmometer in 1850, which was the precursor of the keratometers still used in clinical practice today. However, this development tends to be little recognized, and to be overshadowed by others of the achievements of this singular scientist. This review describes the historical setting behind the von Helmholtz's ophthalmometer and its mechanism. We also describe the modifications that were later made to the design. We report on our experience measuring a living human cornea with one of the last surviving devices in the world. The ophthalmometer by von Helmholtz marked the beginning of an era in the ophthalmology of the late nineteenth century, and although its original design was not broadly used in the clinical practice, and later abandoned, it opened the way for the development of practical systems very similar to the ones that we use even today.