

SENSE OF SIGHT. 643 



because these impressions are, under all ordinary conditions, immediately 

 followed by others upon the same part of the retina, and the new 

 sensation practically obliterates the old one. But, if the instantaneous 

 impression be not followed by a different one, or if it be sufficiently 

 vivid to be perceived, notwithstanding the presence of others, its con- 

 tinuance may be made evident to observation. Thus, in a dark room, 

 if a bright point, like the heated end of a wire, be carried round in a 

 circle with moderate rapidity, the eye follows its movement, as it presents 

 itself successively in different parts of the circle ; the light always ap- 

 pearing at one point only, the rest of the space remaining dark. But 

 if the rapidity of the circular movement be greatly increased, the bright 

 point seems to be drawn out more or less into a curved line; and, when 

 the rate of revolution has attained a very high degree of velocity, it 

 becomes transformed into a continuous circle of light, since the 

 impression made upon the retina, when the end of the wire is at one 

 part of the circle, lasts until it has completed a revolution and again re- 

 turned to the same point. The succession of sparks thrown off rapidly 

 from a knife-grinder's wheel often produce the effect, even by daylight, 

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

 by machinery under a high rate of speed, presents apparently a perfectly 

 smooth edge, the outline of which is formed by the moving points of the 

 teeth ; and the revolving spokes of a carriage wheel, in rapid motion, 

 become confused upon the retina with each other and with the interven- 

 ing spaces, and assume the appearance of a uniform glimmering disk. 



The absolute duration of visual impressions upon the retina has been 

 the subject of various researches, but it is found that its length cannot 

 be expressed by any single number which would be correct for all cases. 

 A brilliant light leaves, on the whole, an impression which lasts longer 

 than that from a feeble one ; but, on the other hand, its relative intensity 

 to the light of surrounding objects diminishes more rapidly, and con- 

 sequently, when it is in motion, a higher degree of velocity is required 

 to produce the appearance of a uniformly bright line. The experiments 

 employed to determine the length of time, during which a luminous 

 impression remains upon the eye without appreciable diminution of 

 its intensity, have been usually those with revolving disks, the surface 

 of which is variegated in sectors of black and white. The rate of revo- 

 lution of the disk being known, as well as the width of the different sec- 

 tors, when the revolving surface presents to the eye the appearance of 

 an absolutely uniform gray tint, the time during which the black or white 

 impressions remain undiininished in strength is readily ascertained. The 

 result obtained, from experiments conducted in this manner, under 

 moderate illumination, gives the duration of perfect visual impressions 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, 

 appearing at the same spot, is illustrated in the optical contrivance 

 known as the Thaumatrope, or magic wheel. It consists of an opaque 



