Three commonly accepted notations for visual acuity are: (1) the Snellen System, which follows an arithmetic progression, (2) the Clason Decimal System, which bears a constant relationship to the Snellen System, and (3) the Snell and Sterling System, which follows a geometrical progression and is commonly referred to as the system of the American Medical Association. Visual acuity is indeed difficult to measure and many types of acuity tests have been devised. One such test, which is administered on the stereoscope, has been devised by Betts, Director of Teacher Education, State Normal School, Oswego, N. Y., who worked jointly with the Keystone View Company of Meadville, Penn. The principle of the Betts tests is the ability to resolve a black clot set within a target when placed in the stereoscope. In tests that were made the Betts acuity test was given simultaneously with the standard Clason acuity tests and curves were plotted showing the relationship between the two. Measurements were made on 5000 persons in connection with automobile drivers' tests given in several states.
Proceedings of the Iowa Academy of Science
©1938 Iowa Academy of Science, Inc.
Schepler, H. C.
"Visual Acuity Measurements with the Betts Test Cards,"
Proceedings of the Iowa Academy of Science, 45(1), 275-276.
Available at: https://scholarworks.uni.edu/pias/vol45/iss1/72