Mariotte was the first to recognize that all parts of the retina were not equally sensitive. According to most authors, a limited portion of this membrane, corresponding to the papilla of the optic nerve, is totally insensible to light M. Longet admits that there it has, at any rate, a very obtuse sensibility. This "blind point" is the only one on the internal surface of the eye which is not covered with pigment.

If we trace two figures on a horizontal line on a piece of paper placed vertically (fig. 36, p. 161), and then shut the right eye, and fix the left on the right figure, at certain distances we can see both of them more or less distinctly; but varying the distance of the paper from the eye, there is a point when we only see the figure upon which the eye is fixed; the other disappearing entirely, but it reappears when we change the position of the paper, or cease to look fixedly at one figure. The greater the distance between the two figures, the greater must be the distance of the paper from the eye, in order to render one of them invisible. The image of the invisible one is then projected on the blind point, and it reappears, when by the displacement of the paper the angle which its rays form with those from the other becomes more or less open.

Fig. 36.

Entoptics

The eye perceives not only external objects, but. certain details of its internal organization. This portion of the visual phenomena is called interior vision, or entoptics. A bruise of the cornea through the lids, or a scar or foreign body on its surface, the vascular ramifications of the retina, and other causes of that nature, sometimes throw images on the retina of different forms, such as striae spots, globules, dark or luminous circles which appear to move in the eye. Certain of these images are called flies, or motes (muscae volitantes), because they traverse the field of vision from one side to the other. Their appearance results from nothing abnormal, and they cannot be confounded, when there is no other disturbance of the visual functions, with the analogous signs which accompany and denote some diseases of the eye and brain. A lateral movement of the eye is sufficient to displace them or cause them to disappear.