Visual Diffusivity, 



23 



where this image merges with the rod image. I£ measured 

 after a red image it becomes equal to the blue-red interval 

 at the same point. 



3. Qualitative Observations on Retarded Images and 

 After-images. 



(a) Apparatus. 



The apparatus, which was designed so as to permit of 

 measurement of the time-interval between two impressions, 

 is shown diagrammatically in fig. 4. It consists essentially 



Fisr. 4. 



Diagram of apparatus for studying interval between perception of 

 images of different brightness and colour. 



of two superposed circular disks of sheet metal, A and B, 

 one 22 cm. the other 36 cm. diameter, both mounted on the 

 same axis, X, arid locked in any relative angular relation 

 by the nut N. In each disk is narrow radial slot, 0, of five 

 degrees opening, the relative positions of these slots being 

 variable from coincidence of direction up to about sixty 

 degrees divergence, as read on the scale S. On the back of 

 the larger disk is mounted a piece of flashed opal glass which 

 is illuminated by the light source behind and is the actual 

 bright surface observed. Grooved holders are provided over 

 Oa and Ob for carrying coloured or other glasses, and the 

 small streak of light between the disks at L is covered by 

 strips of electric wiring tape. The disks are rotated slowly 

 by an electric motor with a multiple reduction pulley system 

 by which speeds of one revolution per second and less are 



