92 EUPHA FOLEY TUGMAN 



On November 21st the difference in intensity between the 

 standard and the variable lights was shifted from .036 c.p. to 

 .022 c.p. This seemed to be too large a step and confused the 

 bird, so on November 28th the difference was placed back at 

 .036 c.p. and the bird soon relearned that discrimination. Then 

 when the difference in intensities was reduced in smaller steps 

 the bird learned to discriminate the lights with differences much 

 smaller than .022. In the descending series the least discrim- 

 inable difference in intensity was .013 c.p. When the intensity 

 was reduced to .009 c.p. the bird seemed wholly incapable of 

 discrimination and very soon fell into the position habit. When 

 the intensity was increased to .013 c.p. the experimenter was 

 unable to break up the bird's position habit though it had dis- 

 criminated the lights at this position in the descending series. 

 Finally the lights were moved still farther back to .017 and after 

 14 days the experimenter succeeded in breaking up the position 

 habit. The bird was again able to discriminate the illuminated 

 areas. Hence the position midway between .013 and .017 c.p. 

 is taken as the least discriminable difference. That position 

 gives the difference in intensity between the standard and the 

 variable as .015 c.p. Of the 599 wrong choices the bird made 

 223 when the brighter light was on the right and 376 when it 

 was on the left. 



Female V did not give as good results. This the observer 

 attributed to the fact that she was always so frightened that 

 she would never hesitate long enough to make a discrimination. 

 She apparently had little position habit but simply went to one 

 side or the other in a confused manner. She made the neces- 

 sary 30 consecutive correct choices with the difference in intensity 

 .036 c.p. But when the position was shifted giving a difference 

 in intensity of .030 c.p. she was not able to discriminate even 

 after 540 trials. So the variable light was moved back, in- 

 creasing the difference to .036 c.p. and after 600 trials she finally 

 made 30 correct choices. Hence the least discriminable differ- 

 ence for Female V is .033 c.p. 



Out of 434 wrong choices 305 were made when the brighter 

 light was on the right and 129 when the brighter light was on 

 the left. 



For Male V and Female VI the problem was very much more 

 difficult. They had to overcome their natural tendency to go 



