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 ;+ 
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 from the discharge of voltaic 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 effect 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 follows, 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 
experiments on this point, as its brightness is inferior to that of sun-light. According to Sir Jonn 
Herscuet, 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, Larpner’s Cyclopedia, p. 210. London, 1835.) 
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. 
+ See Light. Encyclopedia Metropolitana, Art 58. 
{ Experimental Researches in Electricity, vol. 1., sec. 333. Lond. 1839. 

