108 EXPERIMENTAL PHYSIOLOGY 



This experiment is varied by employing colours e.g. a yellow 

 spot on a blue ground or vice versa ; and a red spot on a green ground 

 ot vice versa. In each case the vision is transferred to a uniform white 

 surface and the contrast colours are observed. 



For the grounds coloured paper is used ; for the spots either disks 

 cut out of the paper, or wafers laid upon it. 



Meyer's experiment. Place a grey disk upon a yellow ground, and 

 cover the whole with thin tissue paper ; the grey disk at once appears 

 blue : the contrast colour of the yellow. If a blue ground be used the 

 disk will appear yellow. The same experiment may be repeated with 

 red and green grounds. On a white ground the grey disk 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 fastened. 

 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. 



