It is difficult to define visual acuity, which changes from day to day and from minute to minute. In the discrimination of details, a part is played by physical, physiologic and psychologic factors. In the highest grade of visual discriminations—the form sense—the psychologic factors play a major rôle and obscure the physiologic ones.
Visual acuity varies with the illumination of the object and of the surrounds—the general illumination in which the subject sits—and with the adaptation of the eye.
It was once believed that two points separated by one minute of visual angle appear discrete. The images of these points lie about 0.0043 mm. apart on the retina, slightly more than the diameter of a foveal cone. It was thought that in order for the eye to resolve two dots, their images should have an unstimulated cone between them. This theory is no longer tenable. According to Hofmann (1920), the