FORMATION OF IMAGES IN THE EYE 6/5 



tracted when the light striking the eye is intense and is dilated as the 

 quantity of light is diminished. In accommodation of the eye, the 

 pupil is dilated for distant 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 diaphragm, thus aiding 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 aberration, that the image formed upon 

 the retina is practically perfect. The layer of rods and cones of the 

 retina is the only part in the eye endowed with special sensibility, the 

 impressions of light being conveyed to the brain by the optic nerves. 

 This layer is situated next the pigmentary layer; but the other layers 

 of the retina, through which the light passes to reach the rods and 

 cones, are transparent. 



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

 capable of directly receiving visual impressions, by the following experi- 

 ment, 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 possi- 

 ble 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 appear- 

 ance produced by the shadows of the large retinal vessels ; and as the 

 lens is moved slightly, the shadows of the vessels move with it. With- 

 out going elaborately into the mechanism of this phenomenon, it is suf- 

 ficient to state that Heinrich M tiller has arrived at a mathematical 

 demonstration that the shadows of the vessels are formed on 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 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 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. If a ray of light falls on the border of the 

 cornea at a right angle to the axis of the eye, it is refracted by its sur- 

 face and will pass through the pupil to the opposite border of the 



