THE EYE, AND THE PHENOMENA OF VISION. 359 



The apparent motion of certain colored figures in -worsted work, known by 

 tlie name of the "dancing mice," is due to the fact that when the surface 

 is moved in a particular direction, as from side to side, the impression of the 

 color on the retina remains for an appreciable interval after the figures have 

 moved, and this gives to them an apparent motion. This effect will not, 

 however, take place unless the colors of the figures and the ground-work are 

 very brilliant and complementary of each other, as red upon a green ground. 



When is motion 716. No motion is perceptible to the eye 

 tofureye'?"^ which has a less apparent velocity than one 

 degree per minute. 



It is for this reason that the motions of the heavenly bodies are invisible, not- 

 withstanding their immense velocity. The apparent motion of the sun, moon, 

 and stars, owing to the revolution of the earth, is one quarter of a degree a 

 minute ; but if the earth revolved on its axis in six hours instead of twcnty- 

 foar, then the celestial bodies would have a motion of one degree per minute, 

 and their movements would be distinctly perceptible. 



For the same reason, the motions of the hands of a clock are not per- 

 ceptible to the eye. 



On the contrary, when a body moves with such rapidity from one position 

 to another, that its image does not remain long enough upon one point of the 

 retina to sufficiently impress it, it becomes invisible. Kence it is that a 

 ball discharged from a cannon, and passing transversely across the eye, is not 

 seen. 



How is appa- Apparent motion is affected by distance, and 

 f^LTbT^t t^e motion of a body which is visible at one 

 taace? distancc may be invisible at another, inasmuch 



as the angular velocity will be increased as the distance is 

 diminished. 



Thus, if an object at a distance of 57^ feet from the eye move at the rate 

 of a foot per second, it will appear to move at the rate of one degree per 

 second, inasmuch as a line one foot long at 57^ feet distance subtends an 

 angle of one degree. Now if tlie eye be removed from such an object to a 

 distance of 115 feet, the apparent motion will be half a degree, or thirty min- 

 utes per second ; and if it be removed to thirty times that distance, the ap- 

 parent motion will be thirty times slower. Or if, on the other hand, the eye 

 be brought nearer to the object, the apparent motion will be accelerated ia 

 exactly tlie same proportion as the distance of the eye is diminished. 



A cannon-ball moving at 1,000 miles an hour transversely to ths lino of 

 vision, and at a distance of fiftv yards from the eye, will be invisible, since it 

 will not remain a sufficient time in any one positi?/a to produce perception. 

 The moon, however, moving with more than double the velocity of the can- 

 non-ball, being at a distance of 240.000 miles, has an apparent motion soalow 

 aa to bo imperceptible to the unassisted eye. 



