Survey of the Colour Vision of 1000 Students. 189 



(3) Screw down until you can just see which of the above 

 four objects you have, then say whether the red is 

 above, below, to the right or to the left. Repeat. 



(4) Screw down until you can just see which o£ the above 

 four objects you have, then say whether blue is 

 above, below, to the right or to the left. 



Perhaps the nature of the test will be understood best 

 from the figures for a particular case. The vertical black 

 spots were focussed sharp at 34*5 and the horizontal black 

 spots at 34*3 mm. The mean, 34'4, was taken as the 

 observer's focus. He was then given the horizontal black 

 disks very far out of focus, so that there was nothing visible 

 at all in the field, and screwed slowly down until he could 

 just recognize them. This he did at 48. Similarly he 

 could just recognize the vertical black disks at 49*4. The 

 differences of these two numbers from 34'4, i. e. 13*6 and 15, 

 were taken as a measure of his ability to distinguish black 

 from white. The supervisor next placed a red-green pair in 

 the field far out of focus. The observer looking into the 

 microscope saw nothing at all in the field ; he screwed down 

 and recognized the object as "red left " at 49*7. He was 

 then <ziven another red-green pair and recognized it as "red 

 right'' at 52*5. Consequently his ability to distinguish red 

 from green was measured by 15'3 and 18*1. He was next 

 given a green-blue pair and recognized it as " blue above " 

 ;;t 49. Finally he recognized " blue below " at 45'9, so that 

 his ability to distinguish blue from green was measured by 

 14 - 6 and 11*5. Very often, when he was first given the 

 black disks out of focus, owing to having not fully under- 

 stood the instructions the observer screwed down much too 

 far and asked permission to repeat the determination. This 

 was always granted in the case of the first test with the 

 black disks, but not in the case of the other tests. The test 

 with the black disks was arranged principally for giving 

 familiarity with the apparatus before the colour tests were 

 entered upon. 



As the coloured disks were upon a white background, and 

 as they merged with the background when they were out of 

 focus, the effect of screwing out of focus is simply to mix 

 more and more white with the colour in question, until we 

 have finally, in the case of "red above'' for example, merely 



