S. P. Langley — Energy and Vision. 



377 



The probable error of a series of ten readings of fine print, 

 under the actual conditions of observation with a (feeble) stan- 

 dard luminosity, is determined for two of the observers as 

 follows : 



Yiolet Light 

 a=0/"-40 



Orange-yellow light 

 1 = 0/^-60 



Scarlet light 

 A = 0/"-65 



Probable Error of one observation. 



F. W. V 



% 

 5-53 



% 

 176 



% 

 3-14 



E. M. . 



7-69 



2-51 



2-86 







Probable Error of mean. 



F. W. V 



1-75 



0'56 



0-99 



E. M. 



2-44 



0-80 



090 







The measurements with violet light were made June 19, 1888, " sky hazy blue, 

 thin but uniform cirrus haze." Those at wave-lengfchs 0/" - 60 and OP-Qb were ob- 

 tained on June 20, 1888, " sky hazy blue with cumuli, haze not as dense as on the 

 19th but possibly less uniform." 



For a large part of the spectrum the probable error of a 

 single reading does not exceed 4 per cent but the error may 

 considerably exceed this for the violet rays, the eye requiring 

 a much longer time to regain its sensitiveness for light of this 

 color than for any other, so that for measures in this region an 

 hour's stay in the darkened room is none too much to develop 

 the full power of an eye which has recently been exposed to 

 the full sunshine. 



Time required for Vision. 



In connection with the photometric measures the time re- 

 quired for the perception of very faint colored lights was 



Sensitiveness violet (/"-40) : green (-"-55) = 1 : 240 



scarlet (/"-65) : green (/"-55) = 1 : 230 



" crimson (/"'75) : green (^-55)=1 : 450,000 

 Photometry by the reading of fine print gave for the same observer 



. Yiolet, sensitiveness of eye =0104,000 

 Green, " " =5-790,000 



Scarlet, " " =0036,000 



Crimson, " " =0-000,070 



and the relative effect by this 



unity being the sensitiveness for yellow light 



method is 



Yiolet (P-40) : green (/"-55)=1 

 Scarlet (^65) : green (/"*55)=1 

 Crimson (^-75) : green (/"-55)=1 



56 



160 



83,000 



