602 MR WILLIAM SWAN ON THE GRADUAL PRODUCTION OF 



3. On the Difference between the Apparent and the Intrinsic Brightness of the Flash 

 produced by Electricity of high tension. 



Professor Wheatstone, as was already noticed, has proved that the light of 

 electricity of high tension has a less duration than the millionth part of a second. 

 Now, since it has been shewn that lights of every intensity produce their impres- 

 sions on the eye in equal times,* and that the brightness of an impression is 

 exactly proportional to its duration ; it follows, that if the electric spark could be 

 made to last for the hundredth part of a second, which is 10,000 times its actual 

 duration, its apparent brightness would also be increased 10,000 times. But the 

 results already recorded (see Table, p. 595), shew that the apparent intensity 

 of light lasting for the hundredth part of a second scarcely exceeds a tenth of its 

 real intensity. Hence, if the duration of the electric spark could be prolonged so as 

 to render its light continuous, its apparent brightness would probably be increased 

 about 100,000 times. 



From the nature of the experiments on which this conclusion is founded, it is 

 perhaps only strictly applicable to the case where the electric spark is seen by the 

 eye already acted on by light of moderate intensity ; for in other cases its apparent 

 brightness is no doubt greatly increased by the contrast with previous darkness ;f 

 but however remarkable the conclusion may appear, it seems perfectly consistent 

 with the estimate of the intrinsic brightness of the electric spark, which arises 

 from reflecting on the extremely short space of time in which its powerful im- 

 pression on the eye is produced. 



Dr Faraday observes, that " the beautiful flash of light attending the dis- 

 charge of common electricity, rivals in brilliancy, if it does not even very much 

 surpass, the light fi'om the discharge of A^oltaic electricity;":]: and again he states, 

 that when a battery of 15 jars was discharged through a wet string, " the spark 

 was yellowish, flamy, and having a duration sensibly longer than if the water 

 had not been interposed." Now the eSect of discharging the battery through a 

 bad conductor, would be greatly to diminish the tension of the electricity, while 

 it augmented the duration of the spark. If, therefore, the intrinsic brightness 

 of the spark had remained the same as before, the intensity of the impression on 

 the eye should have been increased ; but the reverse seems to have been the case. 

 Hence it foUows, that the brightness of the electric spark increases with the tension 

 of the electricity. A similar conclusion may obviously be derived from a compa- 



* The electric spark is a light whose intensity places it undoubtedly within the limits of the 

 e.xperiments on this point, as its brightness is inferior to that of sun-light. According to Sir John 

 Herschel, the lime-ball light appears only as a black spot on the disc of the sun when held between 

 it and the eye. — (See Treatise on Astronomy, Lardner's CycloprEdia, p. 210. London, 183.5.) 



I have observed that, in like manner, the spark produced by a strongly-charged Leyden phial, 

 is absolutely invisible when it passes between the eye and the sun's disc. 



■f See Light. Encydopcedia Metropolitana, Art 58. 



I Experimental Researches in Electricity, vol. i., sec. 333. Loud. 1839. 



