To avoid excess complication, weight and cost, the viewer was not designed to be adjustable for various pupillary distances. It was designed with a nominal P.D. of 64 mm and can be used by patients with P.D.'s several millimeters wider or narrower than this, without appreciably restricting the field of view. A wider and a narrower version of the instrument could be designed in the future to cover the full range of patients. The field of view is approximately 20 o , which is sufficient to read a book without needing to turn the head. The introduction of an improved version with a much larger field of view is being considered.
Clinical tests are now underway on a small scale.
I hope that a large scale test can be arranged and the results reported at a future date. Sorsby, Arnold, B. Benjamin, and Michael Sheridan. Refraction and its Components during the Growth of the Eye from the Age of Three . London, H.M. Stationary Office, 1961. Research Council Spec.