124 EXPERIMENTAL PHYSIOLOGY 



red and green grounds. On the white ground the grey disc will appear 

 darker ; on a black ground lighter. 



Colour mixer. This usually takes the form of a revolving circular 

 plate on which sectors of different coloured cards can be arranged. 

 Owing to the fact that retinal impressions have an appreciable dura- 

 tion, the colours appear blended during the revolution of the plate, 

 and the mixing of the colours on the retina can thus be studied. 



Stereoscope. The fact that in stereoscopic vision the mind com- 

 bines the effects produced by slightly dissimilar pictures falling on 

 the two retinae is illustrated by the ordinary stereoscope. 



