THE SENSES. 551 



tion of the spark obtained on breaking the primary current was meas- 

 ured by the aid of an apparatus arranged as in Fig. 141. 



The light emanating from the spark, S, was received by an achro- 

 matic lens, L. It then fell upon a plane mirror revolving with a uni- 

 form velocity of 340 per second, and, after reflection, was brought to a 

 focus upon a glass plate, G, where it could be examined by the eye- 

 piece, E, magnifying ten diameters. 

 From the known rate of revolution FlG - 14*7. 



of the mirror, and its distance from 

 the plate G, the rapidity of motion of 

 the reflected beam upon the plate was 

 determined. If the spark lasted long 

 enough for its reflected image to move 

 ever an appreciable distance, it would 

 appear to be drawn out in a linear 

 fcrm, owing to the persistence of its 

 visual impression. But with the mir- 

 ror revolving at this speed no such 

 alteration was perceptible, the reflected 

 sparkappearingasifstationaryjshow- 



n 



\ t 



ing that the duration Of the light COUld L. Achromatic lens. M. Revolving mir- 



not be greater than .000002 (,*) %? **% 

 of a second. 



In subsequent experiments, there was interposed between the spark 

 and the mirror a glass plate, ruled with alternate transparent and opaque 

 lines, 3 \ of a millimetre in width. Its image, when illuminated by the 

 spark, would appear upon the plate, G, as a series of black and white 

 lines. With the mirror in motion, if the illumination lasted long 

 enough for the image to be shifted a distance equal to the combined 

 width of a black and white line, these lines would become undistin- 

 guishable from each other as in the revolving disk with black and 

 white sectors. Thus the continuance of the visible lines, under a given 

 rate of motion, proved that the duration of the electric spark was less 

 than a certain calculable period. The result showed that the shortest 

 measurable spark lasted but little over .00000004 (sTioTnjToTjO of a 

 second. 



With a spark of this duration, motionless objects were distinctly 

 visible. The letters on a printed page could be recognized, and even 

 the polarization of light was plainly observable. It was accordingly 

 sufficient to produce a complete retinal impression. 



These experiments do not indicate the time required for nervous 

 action in the perception of light. They only show that a luminous 

 impulse having the above duration is sufficient to excite the sensibility 

 of the retina. But the time required for perceiving the sensation is 

 very much longer. From the results given in a preceding chapter 

 (page 371) it appears that the passage of a visual impression through 

 the optic nerve would require at least y^ of a second, and its percep- 



