299 



and of linen, when highly magnified ; and concludes with an histo- 

 rical disquisition on the cloth manufactures of the ancients, and the 

 mention of experiments from which it is inferred that the principal 

 colouring materials employed in dyeing the yarn were indigo and 

 saffron. 



14. (< An Account of some Experiments to measure the Velocity of 

 Electricity, and the Duration of Electric Light." By Charles Wheat- 

 stone, Esq., Professor of Experimental Philosophy in King's College, 

 London. Communicated by Michael Faraday, Esq., F.R.S. 



The continuance for a certain time of all luminous impressions on 

 the retina prevents our accurately perceiving, by direct observation, 

 the duration of the light which occasions these impressions , but by 

 giving the luminous body a rapid motion, which produces the appear- 

 ance of a continued train of light along the path it has described, its 

 condition at each moment may be ascertained, and consequently its 

 duration determined. The same law of our sensations precludes us 

 from direct perception of the velocity with which the luminous cause is 

 moving, as the whole of its track, for a certain distance, appears to be 

 equally illuminated; but by combining a rapid transverse motion of 

 the body from which the light proceeds, with that which it had before, 

 its path may be lengthened to any assignable extent, and both its 

 direction and its velocity will admit of measurement. The author gives 

 various illustrations of this principle, and of his attempts to apply it 

 to appreciate the duration and the velocity of the electric spark. His 

 first experiments were made by revolving rapidly the electric appa- 

 ratus giving electric sparks ; but in every instance they appeared to 

 be perfectly instantaneous. He next resorted to the more convenient 

 plan of viewing the image of the spark reflected from a plane mirror, 

 which, by means of a train of wheels, was kept in rapid rotation on a 

 horizontal axis. The number of revolutions performed by the mirror was 

 ascertained, by means of the sound of a siren connected with it, and still 

 more successfully by that of an arm striking against a card, to be 800 

 in a second. The angular motion of the image being twice as great as 

 that of the mirror, it was easy to compute the interval of time occupied 

 by the light during its appearance in two successive points of its ap- 

 parent path, when thus viewed ; and it was ascertained that the image 

 passed over half a degree (an angle which, being equal to about an 

 inch, seen at a distance of ten feet, is easily detected by the eye,) in 

 the 1,152,000th part of a second. The result of these experiments, 

 as regarded the duration of the spark, was that it did not occupy even 

 this minute portion of time; but when the electric discharge of a 

 battery was made to pass through a copper wire of half a mile in 

 length, interrupted both in the middle, and also at its two extremities, 

 so as to present three sparks, they each gave a spectrum considerably 

 elongated, and indicating a duration of the spark of the 24,000th part 

 of a second. The sparks at both extremities of the circuit were perfectly 

 simultaneous, both in their period of commencement and termination ; 

 but that which took place in the middle of the circuit, though of equal 

 duration with the former, occurred later, by at least the millionth part 



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