Mr Taylor, Interference fringes with feeble light. 115 



remaining times of exposure were taken from the first in the 

 inverse ratio of the corresponding intensities. The longest time 

 was 2000 hours or about 3 months. In no case was there any 

 diminution in the sharpness of the pattern although the plates did 

 not all reach the standard blackness of the first photograph. 



In order to get some idea of the energy of the light falling on 

 the plates in these experiments a plate of the same kind was 

 exposed at a distance of two metres from a standard candle till 

 complete development brought it up to the standard of blackness. 

 Ten seconds sufficed for this. A simple calculation will shew that 

 the amount of energy falling on the plate during the longest 

 exposure was the same as that due to a standard candle burning 

 at a distance slightly exceeding a mile. Taking the value given 

 by.Drude for the energy in the visible part of the spectrum of a 

 standard candle, the amount of energy falling on 1 square centi- 

 metre of the plate is 5 x 10"" ergs per sec. and the amount of 

 energy per cubic centimetre of this radiation is 1*6 x 10~^^ ergs. 



According to Sir J. J. Thomson this value sets an upper 

 limit to the amount of energy contained in one of the indivisible 

 units mentioned above. 



