PECULIAEITIBS OF SIGHT SENSATIONS 465 



layer of cells has received from their shape the name of the 

 rods and cones. They are separated from the choroid 

 layer by a thin layer of pigment. It is these rods and cones 

 which the light vibrations of the projected image stimu- 

 late, and after passing through this complicated system of 

 cells the impulses finally enter the optic nerve fibers and 

 pass on to the brain. A peculiar feature of this arrange- 

 ment is seen in the fact that though the rays pass over the 

 connecting cells to reach the rods and cones they produce 

 no impulses until they reach this area. The rods and cones 

 are believed to convey different qualities of visual impulses. 

 The cones are believed to produce the impulses which give 

 color sensations, while the rods produce merely degrees of 

 light and darkness. 



Peculiarities of the retina. — Not all parts of the retina are 

 equally sensitive to light. The region where the optic nerve enters 

 the eye is a little to one side of the middle line. This spot, of 

 course, has no rods and cones, and objects whose images fall upon 

 this spot are invisible. It is called, therefore, the blind spot. The 

 rest of the retina has a varying sensitiveness. That part of it 

 directly back of the pupil in the mid line, naturally receives con- 

 tinuously more rays than the other parts. At this point (see 

 Fig. 215) is a yellow spot about a quarter of an inch in diameter, 

 where the retina is thinnest, and the rods and cones most numer- 

 ous. This is the region of keenest vision. 



Peculiarities of sight sensations. — Sight sensations are pro- 

 duced almost immediately upon turning the eyes toward an object, 

 but the retina has the power to retain images upon it for a short 

 interval (about one eighth of a second). Owing to this fact, 

 impulses which succeed one another rapidly give the effect of 

 continuous sensation. For example, falling raindrops appear as 

 lines, flames swung in a circle as continuous rings of light, and 

 moving pictures give the effect of a continuous movement. Owing 

 to this same cause, two revolving colors give a mixed sensation, 

 black and white producing gray, etc. 



EDDY. PHYS. — 30 



