| The Demonstration Eye, in the realistic form at the right,
seems not to have been made after the end of the 19th century. The eye
is first adjusted for normal vision by pulling out the brass draw tube.
This has a ground glass screen on one end on which the image of an object
is formed. To simulate near- sightedness, the tube is pulled out, and the
negative corrective lens is swung into place to compensate. Far-sightedness
is simulated by pushing the tube in and using the positive corrective lens.
This example is in the collection of the Smithsonian Institution.
I made a fairly realistic copy of the demonstration eye in the 1980s by using a plastic L'eggs panty hose "egg" for the body of the model, with the sliding tube made from a plastic 35 mm film cassette case with the closed end ground off and replaced by a translucent screen. Positive and negative lenses from an optometrist's set were pivoted in front of the eye lens.
REF: Thomas B. Greenslade, Jr.., "The Demonstration Eye", Phys. Teach., 21, 39 (1983)
The three examples below are from the Smithsonian Institution, Transylvania University and Bates College, reading from left to right.
| This demonstration eye at Transylvania University is somewhat
different from the models above.
"This is an unusual model in that it has a normal lens, a near-sighted lens, and a far-sighted one which may, in turn, be place in the front of the eye. Of course, the near and far lenses produce blurred images on the 'retina' at the rear of the model, but there are corrective lenses which may be placed over the abnormal ones which will then restore the images to a sharp focus. The eye is completely dissectible and may be taken apart for storage in its case." From Leland A. Brown, Early Philosophical Apparatus at Transylvania College (Transylvania College Press, Lexington, Kentucky, 1959) pg 16.
|This eye model, in the Garland Collection of Classical Physics Apparatus at Vanderbilt University, is more devoted to physiology than optics. The model is unmarked, and probably dates from the years before 1900.|
| In the twentieth century the model of the eye was made
of plaster. This example, in regular use at Kenyon College, was
made by Denoyer -[Geppert], "Scientific School Map Makers".
The top lifts off, and the front lens of the eye with its painted iris and the crystalline lens (made of solid glass) may both be removed. The muscles used to turn the eye are clearly shown on the outside, and the fovea centralis and the optical nerve are shown on the inside.
The model looked in about this condition when I first started to use it in 1964, and it probably dates from the nineteen thirties