LIGHT DISCRIMINATION IN THE ENGLISH SPARROW 95 



Female VI had reached a point much nearer the threshold than 

 had Male V after the same number of trials. Male V made 295 

 wrong choices, 184 when the brighter light was on the right and 

 111 when it was on the left. The observer had expected Male V 

 to give better results than any of the other birds because he 

 was in the same cage with the other birds and in the same room 

 where the experiments were conducted for two months and a 

 half before he was experimented upon. He did learn the prob- 

 lem more quickly than Female VI, but then fell behind her 

 after the first change in the intensity of the lights. Male V 

 died before the experiments were finished so his threshold of 

 discrimination was not reached. 



Female VI was a very satisfactory bird with which to work. 

 After she once learned to choose the darker of the two illumi- 

 nated areas, she would always stop just a few seconds outside 

 the door (D,, Fig. 1), look to one side and the other and then 

 hop on calmly to the light she had chosen. She was never 

 excited but always slow in her movements. She worked grad- 

 ually toward the threshold. When the variable light reached 

 43 cm. with the standard at 38 giving a difference of intensity 

 of .022 c.p. she discriminated immediately. The variable light 

 was moved to 42 cm. thus giving a difference in intensities of 

 .017 c.p. At this position she was given 720 trials, with 24 the 

 percentage of error. But she apparently could not discriminate 

 well enough to give two days of perfect trials. So the variable 

 light was shifted back to 43 cm., giving a difference in the in- 

 tensities of the lights of .022 c.p. At this point it took 450 trials 

 before she gave 30 perfect trials in succession, while in the 

 descending series she gave 30 perfect trials after having made 

 only one wrong choice out of the 30 preceding trials. She had 

 evidently been so puzzled with the lights when the difference 

 in intensity was only .017 c.p. that she had either forgotten 

 the problem or had formed the habit of not trying to discrimi- 

 nate. Whatever the cause it took her 450 trials to learn the 

 problem which she had learned in the descending series in 30 

 trials. Out of a total of 414 wrong choices 262 were made when 

 the darker light was on the right and 152 when it was on the left. 



Hence the least discriminable difference for Female VI was 

 .022 c.p., a difference of 5 cm. between the position of the stand- 

 ard and the variable light. 



