Remarks 



poses of succeeding work. Each of the individual 

 striae subtends 4' 14" of visual angle at distance 

 given — 60 cm. 



amenable to statistical treatment. As soon as the problem 

 reaches a certain stage of difficulty, old variable factors, " position- 

 preferences," etc., reappear. It is estimated by students with 

 long experience in this work that the limit of accuracy in deter- 

 mining an animal's threshold is several per cent in either direction. 

 It would appear that such values as are found show the order 

 rather than measure the limit of his ability. 



For the sake of those who are especially interested in a com- 

 parison of the vision of other animals with human vision under 

 similar conditions I have also used some human observers in 

 this work. It was impracticable at this time to resort to the dis- 

 crimination method with them, owing to other work which could 

 not conveniently be interrupted. Using the same test field as 

 was used with these animals, under the same conditions of 

 illumination, and at the same distance from the eye, I tested the 

 threshold of five members of this staff by the method of limits 

 with constant change of stimuli. At each observation the 

 test-bands were made quite large and then gradually reduced, 

 the subject being instructed to announce when they became 

 invisible. They were then made still smaller and increased until 

 the subject announced that they were visible. The mean of 

 these two readings was taken as the threshold for that observa- 

 tion. This procedure was repeated ten times for each subject 

 and the mean threshold determined, the mean variation 

 being taken as indicating the reliability of the determinations as 

 the measure of his visual acuity. The subjects are all attached 

 to the research staff of this laboratory, four being physicists and 

 one a physiologist. All are skilled photometrists. Their results 

 are shown below, along with those obtained on J., a high school 

 student. 



