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