318 



Mr. T. C. Porter. 



[May 14, 



faces AB, ED, and passing through the glass of the two prisms is 

 totally reflected from their hypotenuses AG and EC, then falling on 

 the ground-glass surfaces BC and CD. The prism and the sources of 

 light are so adjusted that the albedos of these faces are equal when the 

 brightnesses of the two sources are as the squares of their distances 

 from C. First, then, for the lowest degrees of illumination, the sperm 

 candle already mentioned was used. This was next replaced by an 

 incandescent electric lamp, of the common type, supplied by a mea- 

 sured and constant current. This gave, as a mean of five determina- 

 tions, the candle-power 25*06. A large coal-gas bat's-wing burner was 

 then measured for candle-power against this incandescent lamp, and, 

 in turn, this gas-burner was balanced photometrically against the arc 

 light which was used for the brightest illuminations, and which gave 

 as the mean of three very careful determinations 1600 candle-power 

 with a possible error of 50 candle-power, though the probable error 

 did not in the writer's estimation exceed 20 candle-power, if as much. 

 The gas-burner was used to connect the measurements of the arc and 

 25'06 candle-power lamp, because the arc and this lamp depended on 

 the same battery of accumulators, and hence could not be compared 

 directly. 



The results of this part of the investigation are exhibited tabularly 

 below,* and in fig. 3 by curves. On the axis of Y distances are measured 

 proportional to the logarithms of the different illuminations of the disc, 

 half white half black, whilst on the axis of X can be read off the corre- 

 sponding number of revolutions of the disc per second in order that 

 flicker may just vanish, and it is at once evident that for all the illumi- 

 nations between that caused by one candle at 2 metres and 12,800 

 times this illumination, the speed with which the disc must be driven in- 

 order that flicker may just vanish varies directly with the logarithm of the 

 illumination of the disc. The same logarithmic law seems to hold good 

 for illuminations smaller than these, but the rate of variation is different 

 though constant. 



With regard to the apparently sudden break in the line, it may at 

 once be said that it may in reality be only a rather sharp curve, but 

 that there is an unexpectedly rapid change in the direction of the line 

 the writer feels certain. Each experiment was repeated at least three 

 times for each observer (of which there were never less than two, and 

 often as many as four), and, moreover, the whole set of experiments 

 were repeated after an interval of 4 years. 



# For this table, see next page. 



