Eyeball Movements in Tests of Visual Acuity

Abstract

IN all published accounts, to which I have access in physiological literature, of testing visual acuity by the discrimination of two points as two and not as one, there is no mention made of a factor which must assuredly play an important part. The three qualities usually considered are the angle subtended, the illumination, and the contrast between the points viewed and their background. In all the discussions which I have read it is tacitly assumed that the eyeball is fixed. Now we know that a limb, and the eyeball is physiologically a limb, cannot be kept in a position of rest unless it is at an extreme of movement, as when the knee is fully extended or when gravity alone determines the posture. The best marksman is aware of the tremor of his rifle when it is supported solely by muscular action; indeed physiological theory demands that such tremor must exist. By no possibility can we assume that the eyeball, held in any ordinary position by muscular action, is free from such tremor. The fact that tremor is not obvious does not proclaim its absence; when it is obvious we speak of it as nystagmus. Microscopic examination of the pupil tells us that the inner iridic margin, to the observing naked eye apparently fixed, is really in a state of tremor of small amplitude. The only recognition of the possibility of this movement of the eyeball which I have found is an article “The Application of the Physiology of Color Vision in Modern Art”, by Henry G. Keller and Prof. J. J. R. Macleod in Popular Science Monthly, November 1913: