PERCEPTION OF A MOVING OBJECT. 



feet distance subtends an angle of one degree. Now if the eye be 

 removed from such an object to a distance of 115 feet, the apparent 

 motion will be half a degree, or thirty minutes per second ; and if 

 it be removed to thirty times that distance, the apparent 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 acceler- 

 ated in exactly the same proportion as the distance of the eye is 

 diminished.. 



60. A cannon-ball moving at 1000 miles an hour transversely 

 to the line of vision, and at a distance of 50 yards from the eye, 

 will be invisible, since it will not remain a sufficient time in any 

 one position to produce perception. The moon, however, moving 

 with more than double the velocity of the cannon-ball, being at 

 a distance of 240000 miles, has an apparent motion, so slow as 

 to be imperceptible, 



61. The angular motion of the line of vision may be so dimin- 

 ished as to become imperceptible ; and the body thus moved will 

 in this case appear stationary. It is found by experience that 

 unless a body move in such a manner that the line of vision 

 shall describe at least one degree in each minute of time, its 

 motion will not be perceptible. 



62. Thus it is that we are not conscious of the diurnal motion 

 of the firmament. If we look at the moon and stars on a clear 

 night, they appear to the eye to be quiescent ; but if we observe 

 them after the lapse of some hours, we find that their positions are 

 changed, those which were near the horizon being nearer the 

 meridian, and those which were in the meridian having descended 

 towards the horizon. Since we are conscious that this change did 

 not take place suddenly, we infer that the entire firmament must 

 have been in continual motion round us, but that this motion is 

 so slow as to be imperceptible. 



Since the heavens appear to make a complete revolution in 

 twenty-four hours, each object on the firmament must move at 

 the rate of 15 an hour, or at the rate of one quarter of a degree 

 a minute. But since no motion .is perceptible to the eye which has 

 a less apparent velocity than 1 per minute, this motion of the 

 firmament is unperceived. If, however, the earth revolved on its 

 axis in six hours instead of twenty-four hours, then the sun, 

 moon, stars, and other celestial objects, would have a motion at 

 the rate of 60 an hour, or 1 per minute. The sun would appear 

 to move over a space equal to twice its own diameter each minute, 

 and this motion would be distinctly perceived. 



The fact that the motion of the hands of a clock is not perceived 

 is explained in the same manner. 



63. If an object which moves very rapidly be not sufficiently 



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