﻿6±6 Miss M. 0. Saltmarsh on the 



pressure the adsorbed fluid assumes the liquid state, and the 

 aggregation develops, by the rapid condensation which 

 ensues, into the large ion of Langevin. 



The intermediate and large ions thus appear to form 

 a somewhat striking illustration of Trouton's discovery of 

 the two modes of condensation of water-vapour on rigid 

 surfaces. 



The Physical Laboratory. 



The University of Sydney, 

 January 8, 1915. 



LXYIII. The Brightness of Intermittent Illumination. By 

 M. 0. Saltmarsh, B.A., Demonstrator in Fhysics at Bedford 

 College^ London*. 



WHEX light from a constant source falls on a surface, 

 the illumination over a small area is uniform. If 

 the source is screened from the surface at regular intervals 

 of time, the illumination will appear intermittent and flicker- 

 ing, unless the interval of time during which the light is 

 screened, be small enough. For a particular value of this 

 interval of time, the flickering of the illumination will just 

 cease, and in this case the visual impression received when 

 the screen is illuminated will be just carried on with equal 

 intensity over the period of darkness, so that the illumination 

 is apparently uniform. If the time of complete darkness is 

 less than this value, and the visual impression which is carried 

 on from one time of illumination overlaps that received 

 during the next time of illumination, the eye, while still 

 retaining the effects of one stimulus, will be acted on by 

 another. 



Is the brightness, therefore, that corresponding to the 

 new stimulus alone ? or, does the residual effect of the pre- 

 ceding one influence it in any way ? 



It was with a view to settling this point that the observa- 

 tions described in this paper were made. 



A photometer bench, about 2*5 metres long, was used and 

 it was fitted with a smoothly running upright carrying a 

 Bunsen photometer. An electric lamp was placed at either 

 end of the bench, and in front of one, a black cardboard disk 

 was rotated about a horizontal axis through its centre by 

 means of an electric motor. 



The number of revolutions of the disk per second was 



* Communicated by Sir J. J. Thomson, O.M., F.K.S. 



