THE EYE AS AN OPTICAL INSTRUMENT. 205 



tween 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 disappear- 

 ance of the visual purple, and we can not see at all until the pur- 

 ple 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 per- 

 son 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 mechan- 

 ism of accommodation. The area outside of this is used for indis- 

 tinct 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 retinae. 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, 



