114 Mr Taylor, Interference fy^wges ivith feeble light. 



Interference fringes with feeble light. By G. I. Taylor, B.A., 

 Trinity College. (Communicated by Professor Sir J. J. Thomson, 

 F.R.S.) 



[Bead 25 January 1909.] 



The phenomena of ionisation by light and by Rontgen rays 

 have led to a theory according to which energy is distributed 

 unevenly over the wave-front (J. J. Thomson, Proc. Camb. Phil. 

 Soc. XIV. p. 417, 1907). There are regions of maximum energy 

 widely separated by large undisturbed areas. When the intensity 

 of light is reduced these regions become more widely separated, 

 but the amount of energy in any one of them does not change ; 

 that is, they are indivisible units. 



So far all the evidence brought forward in support of the 

 theory has been of an indirect nature ; for all ordinary optical 

 phenomena are average effects, and are therefore incapable of 

 differentiating between the usual electromagnetic theory and the 

 modification of it that we are considering. Sir J. J. Thomson 

 however suggested that if the intensity of light in a diffraction 

 pattern were so greatly reduced that only a few of these indivisible 

 units of energy should occur on a Huygens zone at once the ordinary 

 phenomena of diffraction would be modified. Photographs were 

 taken of the shadow of a needle, the source of light being a 

 narrow slit placed in front of a gas flame. The intensity of the 

 light was reduced by means of smoked glass screens. 



Before making any exposures it was necessary to find out what 

 proportion of the light was cut off by these screens. A plate was 

 exposed to direct gas light for a certain time. The gas flame was 

 then shaded by the various screens that were to be used, and other 

 plates of the same kind were exposed till they came out as black 

 as the first plate on being completely developed. The times of 

 exposure necessary to produce this result were taken as inversely 

 proportional to the intensities. Experiments made to test the 

 truth of this assumption shewed it to be true if the light was 

 not very feeble. 



Five diffraction photographs were then taken, the first with 

 direct light and the others with the various screens inserted 

 between the gas flame and the slit. The time of exposure for the 

 first photograph was obtained by trial, a certain standard of 

 blackness being attained by the plate when fully developed. The 



