SENSE ORGANS. j,r 



blind spot can be perceived in the field of darkness. At 

 night in the dark — when the retina by long rest is very 

 sensitive — if the visual plane be lowered toward the feet, 

 and then the eyes be turned quickly and strongly to one 



Fig. 89. — Diagram showing place of the invisible spots in the field of vision. 

 The full lines show the eyes turned to the right ; the dotted lines the 

 same turned to the left. Ps = point of sight. 



side or the other, two brilliant stars with dark centers 

 are seen to flash out for a moment in the dark field 

 (Fig. 89). The phenomenon is produced by a pull on the 

 optic nerves. The dark center is the spatial representa- 

 tive of the blind spot, and the brilliant radiating circle is 

 produced by irritation of the surrounding bacillary layer. 



COLOR PERCEPTION AND COLOR-BLINDNESS. 



We have thus far treated of perception of light only 

 as intensity and direction. But another primary per- 

 ception is that of color. 



