THE EYE AND THE PHYSIOLOGY OF VISION. 577 



around on a string looks like a luminous circle ; a point of 

 light moving rapidly up and down looks like a luminous 

 stick. All this is explained on the ground that the sensa- 

 tions linger and that we therefore see the rocket in a cer- 

 tain position after it has left it and attained a new one. 

 We see a rapidly moving luminous object in the position in 

 which it is at the moment, together with quite a number 

 of positions through which it has just passed, and in that 

 way its path becomes luminous. A rapidly rotating wheel 

 shows none of the spokes distinctly for similar reasons. 

 If, in a room lighted by a gas jet, the gas be suddenly 

 turned off while one is looking at it, an image of the flame 

 remains an instant or two after that on the retina. Pos- 

 sibly the most familiar illustration of this fact is in the 

 lightning, a stroke of which seems to us to extend in a 

 continuous although zig-zag line from the cloud to the 

 earth. 



Positive after-images retain not only the shape, but also 

 the color of the object and offer no serious difficulty in their 

 explanation. It is not so easy with negative after-images. 



2. Negative After- Images. If one glances at the sun 

 just an instant and then looks up into the sky in a differ- 

 ent direction one sees a round black disk. If one looks in- 

 tently with the eyes fixed on a red light and then shifts 

 his view to a white wall an image of the light looked at is 

 projected against the wall but in different colors, usually 

 bluish or greenish. Or, if one looks out of the window 

 intently for some time, then turns the look to the wall, 

 the white window panes are black, the black window 

 frames somewhat light in the negative after-image. An 

 after-image retains the form and size of the real image, but 

 in its complementary colors. A very clear illustration of 

 this on a wider scale is afforded by placing close before the 

 eye an intensely red light and looking at the same for some 

 time, even at the risk of tiring the eye. Upon turning now 

 to a room lighted with electricity or gas light, or even ordi- 

 nary day light, the objects possess an intensely bluish tint, 

 37 



