Page:Popular Science Monthly Volume 45.djvu/219

Rh the rods and discharge a liquid which colors the rods alone. When the rods are thus colored, the eye is extremely sensitive, so that a bright light is dazzling and painful and obscures distinct vision. This is the reason why we can not see distinctly when we come suddenly from the dark into a full light. In a few seconds, however, the color is bleached to a yellow and the difficulty passes away. When, on the other hand, we pass from a bright light into the dark, the retina has lost its sensibility from disappearance of the visual purple, and we can not see at all until the purple is reproduced, as it is in the absence of light. This difference is not due to dilatation of the pupil in the dark and contraction under the influence of light, as is popularly supposed, for a person does not see better in the dark when the pupil has been fully dilated by belladonna.

In the little area of distinct vision there is never any visual purple. This area we always use with sufficient light for minute details of objects, making then the greatest use of the mechanism of accommodation. The area outside of this is used for indistinct vision, and as the color is then yellow instead of purple, it is only moderately sensitive. To express the conditions in a few words, the minute area for distinct vision is used by day, and the area for indistinct vision, with its visual purple, is used by night.

A very curious condition is what is known as night-blindness. Sometimes, in long tropical voyages, sailors become affected with total blindness at night, while vision in the daytime is perfect. The glare of the sun in the long days bleaches the visual purple so completely that it can not be restored in a single night, and the area of indistinct vision becomes insensible. This trouble is purely local and is remedied by rest of the eye. If one eye be protected by a bandage during the day, this eye will be restored sufficiently for the next night's watch, while the unprotected eye is as bad as ever. Snow-blindness in the arctic regions is due to the same cause.

We receive the impression of a single object, although there are two images one in either eye; but it is necessary that the images be made upon corresponding points in the two retinæ. If the angle of vision in one eye be deviated even to a slight degree by pressing on one globe with the finger, we see two images. One can appreciate how exactly these points must correspond when it is remembered that two rays of light appear as one only when the distance between them is one thirty-five-hundredth of an inch.

In either eye there is a blind spot, and this is at the point of penetration of the optic nerve; but, inasmuch as this spot is in the area of indistinct vision, and is so situated a little within the line of distinct vision that an impression is never made on both blind spots by the same object, this blindness is never appreciable,