1. A device for measuring visual acuity both under normal conditions as well as following medical or visual stress comprising means for projecting a moving spot of light in the view of a subject, means for attenuating the intensity of said spot of light, eye tracking means for sensing the position of the eyes of the subject while said spot of light is present, difference means for comparing the position of the eyes of the subject and the position of said spot of light, including detector means for sensing divergence therebetween, converter means for converting said intensity of said spot of light at the instance of said divergence, to visual acuity units, and display means for displaying the output of said converter means.