The Sense of Sight: Brightness Vision 127 



To test the value of my supposition, I made additional 

 experiments with the 20 standard, the results of which ap- 

 pear under the dates June 2 and 3 of the table. These re- 

 sults indicate quite definitely that the animal had been, and 

 still was, improving in her ability to discriminate. For in- 

 stead of requiring a difference of about one half in order that 

 she might distinguish the 20 standard from the variable 

 light she was now able to discriminate with only 22 per cent 

 of errors when the difference was one tenth. 



As it seemed most improbable that improvement by train- 

 ing should continue much longer, I next gave additional tests 

 with the 80 standard. Again a difference of one tenth was 

 sufficient for accurate discrimination (18 per cent of errors). 

 These series were followed immediately by further tests 

 with the 5 standard. As the results indicated greater ease 

 of discrimination with a difference of one tenth in the case 

 of this standard than in the case of either of the others I was 

 at first uncertain whether the results which I have tabulated 

 under the dates June 3, 4, and 5 of the table should be in- 

 terpreted in terms of Weber's law. 



Up to this point the experiments had definitely established 

 two facts : that the dancer's ability to discriminate by means 

 of brightness differences improves with training for a much 

 longer period and to a far greater extent than I had supposed 

 it would ; and that a difference of one tenth is sufficient 

 to enable the animal to distinguish two lights in the case of 

 the three standard values, 5, 20, and 80 hefners. The ques- 

 tion remains, is this satisfactory evidence that Weber's law 

 holds with respect to the brightness vision of the dancer, or 

 do the results indicate rather, that this difference is more 

 readily detected in the case of 5 as a standard (12 per cent 

 error) than in the case of 20 as a standard (22 per cent 

 error) ? 



