198 Dr. Houstoun and Miss Dunlop on a Statistical 



curve in the case of fig. 9 is given by the same expression, 

 where ?/ = 24*29, a = 7'02Q, ??i = 4*101, and the origin is at 

 15*33. The smooth curve in the case of fig. 10 is a Gaussian 

 one, cr being =3' 12 9, while the origin is at 16*06 and 

 z/ = 21*04. The observer shown at 1*5 in fig. 9 was examined 

 by the bead test and appeared normal. P has the values "40, 

 •75, and -74 for figs. 8, 9, and 10 respectively. Sheppard's 

 adjustments were not used except for fig. 5 and ordinates 

 were taken instead of areas except for fig. 7, so some of the 

 values of P obtained may be improved upon. 



Thirteen men obtained a value less than 7 in the red- 

 green test. The mean value of their readings for the black 

 is 10*95 with a probable error of + 1*83. The mean value 

 of all the men for the black was 13*022. The mean value of 

 the same thirteen observers for the blue-green test was 10*61 

 with a probable error of +1*97. The mean value for all 

 the men was 15*547. Hence the red-green colour blind are 

 decidedly below the average in their ability to distinguish 

 blue from green, and somewhat below the average in their 

 ability to distinguish black from white. The numbers 

 obtained by these thirteen observers for the black test were 

 plotted against their numbers for the red-green test, as were 

 also the numbers obtained for the blue-green test. But there 

 was no correlation visible from the diagram. 



The following observations were obtained by some of the 

 colour-blind, all men. The number in brackets is the 

 observer's number, then come in order the final corrected 

 figure for the black, red-green, and blue-green test. 



(200) 13*55 15 - 5 Failed in bead test. Could not distinguish red 



from gi*een at focus. 

 Failed in bead test. 

 Normal iu bead test. 

 Mistakes in bead test, 

 formal in bead test. 



Failed in bead test, put bright red in green hole. 

 Said he had difficulty in distinguishing blue 



from green. 

 Had been rejected by Admiralty lantern test. 

 Said at first both red and green spots were green. 

 Said red spot was black. 

 Said he recognized red as the darker colour. 



From previous tests made by Dr. Houstoun it has been 

 found that quite 4 per cent, of men have difficulty in distin- 

 guishing blues from greens, not doubtful cases like peacock- 

 blue, but quite decided tints. This would include the 

 observers in fig. 6 up to 11 mm. 



(211) 



14 



2-95 



15-23 



(246) 



13-25 



2-99 



10-15 



(269) 



11-05 



1-99 



13-59 



(316) 



14-4 



6-2 



10-7 



(387) 



18-1 



12-5 



16-24 



(476) 



1215 



10-25 



13-86 



(554) 



9-45 



4-01 



12-33 



(579) 



11-2 



1-2 



10-83 



(634) 



96 



-1-11 



8-54 



(694) 



15 



11-79 



14-6 



