24 



Dr. H. E. Ives 



obtainable. Speeds are measured by an electric tachometer. 

 An elaborate screening system is provided so that no light 

 gets around the edge of the disks, and observations are made 

 at one metre distance in a darkened chamber. 



The principle which the apparatus is planned to utilize 

 is a familiar one. If the slot Oa be imagined covered with 

 a glass to reduce its intensity, or with a coloured glass of a 

 hue more towards the blue than that of Ob, the theory just 

 developed predicts that on rotating the disks in the counter- 

 clockwise direction, with the slots exactly in line, they will 

 be seen not in line, but the slot Oa will appear to lag behind 

 or be displaced. If this occurs, then it will be possible to 

 move the disk B in the clockwise direction until on rotation 

 the two slots again appear in line. Knowing the angle by 

 which the slots have been turned with respect to each other, 

 and the angular velocity of the disks, it is easy to calculate 

 the interval of time by which one impression is delayed 

 behind the other. 



Before attempting quantitative measurement of these 

 phenomena they were studied in detail qualitatively, and a 

 description of these observations follows : — 



(b) Light and dark stimuli of the same colour. 



The slot Oa is covered by a neutral tint glass, the two 

 slots being in line, and the disks are slowly rotated. At a 

 fairly low intensity of illumination (illumination and other 



Fm. 5. 



Appearance of adjacent dark and light apertures, or blue and red 

 apertures: ^stationary; b, in motion. 



constants are given in the description of the measurements) 

 the dark slot appears several degrees behind the other, 

 presenting the appearance shown in fig. 5 6. At high 



