550 THE NERVOUS SYSTEM. 



cessation of the light. The persistence of these impressions is not usu- 

 ally noticeable, because they are immediately followed by others on the 

 same part of the retina, and are thus practically obliterated. But, if 

 the momentary impression be not at once followed by a different one, 

 or if sufficiently vivid to be perceived, notwithstanding the presence of 

 others, it may be made evident to observation. If a bright point, like 

 the heated end of a wire, be carried round in a circle in a dark room 

 with moderate rapidity, the eye follows it throughout. But if the ra- 

 pidity of its movement be increased, it appears drawn out more or less 

 into a curved line ; and, when moving with very high velocity, it be- 

 comes transformed into a continuous circle of light, since its impres- 

 sion upon the retina, when at one part of the circle, lasts until it has 

 completed its revolution and returned to the same point. The sparks 

 thrown off in rapid succession from a knife-grinder's wheel produce the 

 effect of an unbroken stream of fire. A circular saw with large teeth, 

 revolving under high speed, presents apparently a smooth edge, formed 

 by the moving points of the teeth ; and the spokes of a rapidly-turning 

 wheel become confused upon the retina with the intervening spaces, 

 and assume the appearance of a glimmering disk. 



The duration of visual impressions cannot be expressed by any single 

 term which would be correct for all cases. A brilliant light leaves, on 

 the whole, a longer impression than a feeble one ; but, on the other 

 hand, its relative intensity to surrounding objects diminishes more 

 rapidly, and it consequently requires, if in motion, a higher velocity 

 to produce the appearance of a uniform bright line. The time during 

 which luminous impressions remain, without appreciable diminution 

 of their intensity, is usually tested by means of revolving disks, varie- 

 gated in equal sectors of black and white. The rate of revolution being 

 known, as well as the width of the sectors, when the revolving surface 

 presents a uniform gray tint, the time during which the visual impres- 

 sion remains undiminished is readily calculated. The result of such 

 experiments gives the duration of undiminished impressions, for revolv- 

 ing disks under moderate illumination, as one-twenty-fourth of a second ; 

 and, for the oscillation of a very luminous point following the vibrations 

 of a tuning-fork, one-thirtieth of a second. 



The persistence and apparent continuity of successive visual images 

 are illustrated by the Thaumatrope and other similar contrivances, in 

 which a number of pictures, representing the same object in different 

 positions, are made to pass in quick succession before the eye. Tho 

 intervals between them are too short to be observed, and the figure 

 appears as if in motion. 



Duration of a Luminous Impulse necessary for its Perception 

 This point has been investigated by Rood* by means of the electric 

 spark from an induction coil connected with a Leyden jar. The dura- 



* American Journal of Science and Arts. New Haven, September, 1871. 



