698 SPECIAL SENSES. 



FORMATION OF IMAGES IN THE EYE. 



It is necessary only to call to mind the general arrangement of the differ- 

 ent structures in the eye and to apply the simple laws of refraction, in order 

 to comprehend precisely how images are formed upon the retina. 



The eye corresponds to a camera obscura. Its interior is lined with a 

 dark, pigmentary membrane (the choroid), the immediate action of which is 

 to prevent the confusion of images by internal reflection. The rays of light 

 are admitted through a circular opening (the pupil), the size of which is 

 regulated by the movements of the iris. The pupil is contracted when the 

 light striking the eye is intense, and is dilated as the quantity of light is 

 diminished. In the accommodation of the eye, the pupil is dilated for dis- 

 tant objects and contracted for near objects ; for in looking at near objects, 

 the aberrations of sphericity and achromatism in the lens are more marked, 

 and the peripheral portion is cut off by the action of this movable dia- 

 phragm, thus aiding the correction. The rays of light from an object pass 

 through the cornea, the aqueous humor, the crystalline lens and the vitreous 

 humor, and they are refracted with so little spherical and chromatic aberra- 

 tion, that the image formed upon the retina is practically perfect. The layer 

 of rods and cones of the retina is the only portion of the eye endowed directly 

 with special sensibility, the impressions of light being conveyed to the brain 

 by the optic nerves. This layer is situated next the pigmentary layer of 

 the choroid, but the other layers of the retina, through which the light 

 passes to reach the rods and cones, are perfectly transparent. 



It has been shown that the rods and cones are the only structures capa- 

 ble of directly receiving visual impressions, by the following experiment, 

 first made by Purkinje : With a convex lens of short focus, an intense light 

 is concentrated on the sclerotic, at a point as far as possible removed from 

 the cornea. This passes through the translucent coverings of the eye at this 

 point, and the image of the light reaches the retina. In then looking at a 

 dark surface, the field of vision presents a reddish-yellow illumination, with 

 a dark, arborescent appearance produced by the shadows of the large retinal 

 vessels ; and as the lens is moved slightly, the shadows of the vessels move 

 with it. Without going elaborately into the mechanism of this phenomenon, 

 it is sufficient to state that Heinrich Miiller has arrived at a mathematical 

 demonstration that the shadows of the vessels are formed upon the layer of 

 rods and cones, and that this layer alone is capable of receiving impressions 

 of light. His explanation is generally accepted and is regarded as positive 

 proof of the peculiar sensibility of this portion of the retina. 



Theoretically, an illuminated object placed in the angle of vision would 

 form upon the retina an image, diminished in size and inverted. This fact 

 is capable of demonstration by means of the ophthalmoscope ; as with this 

 instrument the retina and the images formed upon it may be seen during life. 



All parts of the retina, except the point of entrance of the optic nerve, 

 are sensitive to light ; and the arrangement of the cornea and pupil is such, 

 that the field of vision is, at the least estimate, equal to the half of a sphere. 



