190 Dr. Houstoun and Miss Dunlop on a Statistical 



a white field, with a reddish tinge near its centre and 

 immediately below this a greenish tinge. Each observer 

 took about seven minutes in all. The tests were made 

 between January 1919 and March 1920. Of the observers, 

 one was a member of the University staff, 336 were arts or 

 science men students, 84 were arts or science women stu- 

 dents, 261 were first year men medical students, 79 were 

 first year women medical students, and the remainder 239, 

 237 men and 2 women, were first year Technical College 

 students, whom we were able to test through the courtesy 

 of Prof. Muir and the staff of the Natural Philosophy 

 Department of the Technical College. All the tests were 

 made in the course of the student's work in the physical 

 laboratory; each man simply left the optical bench or 

 spectrometer or whatever experiment he was working at, 

 and then returned to it again when the test was completed. 

 Only owing to the large number of demobilised men in the 

 laboratory courses was it possible to carry through the tests 

 with so little trouble; the students were older and, especially 

 the medicals, more capable than usual, and such a favourable 

 opportunity for making such a survey will probably not 

 occur again. The numbers are not the full numbers in the 

 laboratory courses. Some students w r ere omitted through 

 absence or through their not reaching the optical laboratory; 

 the observers were not selected in any way, but just taken 

 as they came. One student declined when asked to make 

 the test, as his experiment was at a critical stage at the 

 time, and as it was the last week in the session no opportunity 

 occurred to test him again, but his was a solitary case. No 

 one else declined. 



The observer could not see the test object except through 

 the microscope. In the tests with the black disks the 

 horizontal spots w r ere sometimes given twice and the vertical 

 spots not at all, or the vertical spots might be given twice 

 and the horizontal not at all, but in the case of the coloured 

 disks it was always a different object each time. 



After the tests were completed we determined the average 

 distances from focus at which each of the ten test objects had 

 been recognized. They were : black horizontal 13*00 mm., 

 black vertical 12-83, red left 15-24, red above 14*89, red 

 below 15*52, red right 16'13, blue left 15'70, blue above 

 15-44, blue below 13*59, blue right 13'76. Thus all the 

 objects had not been equally difficult: to allow for this the 

 readings for red right were decreased in the proportion 16*13 

 to 15*2, those for blue below increased in the proportion 

 13*59 to 15*6, and blue right in the proportion 13*76 to 15*6. 



