188 Dr. Houstoun and Miss Dunlop on a Statistical 



objects ; the test objects are represented in fig. 4. The test 

 objects consisted of pairs of circular disks, which might 

 either be black or red or green or blue. These disks were 



Fig. 4. 



BLACK 



RED 



BLUE GREEN 



each slightly less than one millimetre in diameter. They 

 were drawn with a pen all on one piece of cardboard, the 

 black ones in Indian ink and the coloured ones in Dove's 

 water-proof inks. The piece of cardboard measured 42 mm. 

 by 15 mm., was protected by a glass cover, and was mounted 

 on a strip of wood. The red ink, Dove's carmine, and the 

 green ink, Dove's deep green, were used at the same strength 

 as they came out of the bottle ; the blue, Dove's Prussian 

 blue, was too dark when used at full strength, and was con- 

 sequently diluted with water so as to have the same luminosity 

 as the other two colours. The observer's colour vision was 

 measured by the distance out of focus at which he could 

 just distinguish the different colours. 



Before commencing the test the observer was required to 

 read the following instructions which were typewritten on a 

 piece of cardboard with the disks entered in their appropriate 

 colours : — 



(1) First focus the two vertical black spots 



as 



sru 



two 



. • p 



horizontal black 



as possible. Next focus the 

 spots as sharp as possible $®. 

 (2) Screw down until you can just say whether the object 

 is the two horizontal or the two vertical black spots. 

 Repeat. In this and the following tests the micro- 

 scope is to be as far out of focus as possible consistent 

 with your distinguishing the one case from the other. 

 The distance from focus is to measure the excellence 

 of your vision. 



