The Sense of Sight: Brightness Vision 121 



only one such series was given per day, but sometimes one was 

 given in the morning and another in the afternoon of the 

 same day. To prevent choice by position the lights were re- 

 versed in position irregularly, first one, then the other, illu- 

 minating the right electric-box. For the fifty tests of each 

 initial series the order of the changes in position was as 

 follows: standard (brighter light) on the / (left), /, r (right), 

 r, /, /, r, r, /, r, /, r, /, /, r, r, /, /, r, r, /, /, /, r, r, r, /, r, /, r, r, 

 r, /, /, /, r, r, r, /, /, r, /, r, /, r, /, r, I, r, I. Twenty- five times in 

 fifty the standard light illuminated the right electric-box, and 

 the same number of times it illuminated the left electric-box. 

 When a second series was given under the same conditions 

 of illumination, a different order of change was followed. 



In order to discover whether Weber's law holds in the case 

 of the brightness vision of the dancer it was necessary for me 

 to determine the just perceivable difference between the stan- 

 dard and the variable lights for two or more standard values. 

 I chose to work with three values, 5, 20, and 80 hefners, and 

 I was able to discover with a fair degree of accuracy how much 

 less than 5, 20, or 80 hefners, as the case might be, the vari- 

 able light had to be in order that it should be discriminable 

 from the other. For the work with the 5 hefner standard 

 I used 2-candle-power lamps, 1 for the 20, 4-candle-power, 

 and for the 80, i6-candle- power. 



1 1 give merely the commercial markings of the lamps. They had been 

 photometered carefully by two observers by means of a Lummer-Brodhun 

 photometer and a Hefner amyl acetate lamp previous to their use in the 

 experiment. For the photometric measurements in connection with the 

 Weber's law tests I made use of the Hefner lamp with the hope of attain- 

 ing greater accuracy than had been possible with a standard paraffine candle, 

 in the case of measurements which I had made in connection with the 

 experiments on color vision that are reported in Chapters IX and X. The 

 Hefner unit is the amount of light produced by an amyl acetate lamp at a 

 flame height of 40 mm. (See Stine's " Photometrical Measurements.") A 

 paraffine candle at a flame height of 50 mm. is equal to 1.2 Hefner units. 



