204 H. M. JOHNSON 



we assume a similar "contrast-effect" for the monkey, we have 

 a sufficient explanation of his behavior in these control tests. 



SUMMARY 



Three animals — a monkey and two chickens — previously 

 trained under comparable conditions, showed a striking dif- 

 ference in their individual ability to acquire a perfect habit of 

 response to difference in direction in two systems of visible 

 striae. The monkey perfected his habit in the first series of 

 20 trials. One chick failed to learn the problem and another 

 chick required 58 days and 585 trials to perfect the habit. 



The work demonstrated the practicability of an alternative 

 method of testing visual acuity, but suggested that for some 

 animals the problem is so difficult as to make the method orig- 

 inally used by the author more desirable. 



The monkey's difference-threshold for direction of elements 

 of a pattern lies between 2° and 5°; and the chick's threshold 

 between 25° and 40°. The magnitude of the thresholds within 

 these limits for either individual depends in large part on the 

 effect of training. 



The training-records, as well as the final results, suggest that 

 the characteristic of direction in visual objects has much more 

 significance for the monkey than for the chicken. 



The relative improvement brought about by training is very 

 much greater for the monkey than for the chicken. This fact 

 suggests that other differential factors than relative development 

 of the sense-organ play an important part in discrimination of 

 this type. The point is emphasized by the fact that the mon- 

 key's visual acuity, as shown in an earlier study, is between 

 four and five times as good as that of the chick; while his sen- 

 sitivity to difference in direction is from eight to twelve times 

 as great. 



