The testing of visual acuity and the measurement of ametropia by the improved vision obtained by testlenses are two distinct processes, which will be best carried out by different forms of apparatus. In testing visual acuity we need only to determine a single "minimum separabile," which is done most simply by finding the greatest distance at which a test of known visibility can be recognized. In the subjective measurement of ametropia we have to provide for a "minimum separabile" progressively diminishing as the accurate correction of the refractive error is more nearly approximated. This requires a series of test-objects progressively diminishing in size, such as the ordinary test-cards furnish.
STANDARD OF VISUAL ACUITY
The adoption last year at the Naples ophthalmologic congress of the report of the committee appointed at the preceding ophthalmologic congress at Lucerne to propose an international standard of normal visual acuity, with the discussions of the
JACKSON E. TESTS OF VISUAL ACUITYAND CARDS FOR SUBJECTIVE CORRECTION OF AMETROPIA. JAMA. 1910;55(6):472-475. doi:10.1001/jama.1910.04330060024007