PRACTICAL EXERCISES 



1069 



of a millimetre. Set the card up in a good light, and walk backwards 

 from it till the individual lines just fail to be discriminated. Measure 

 the distance from the card at which this occurs, and calculate the size 

 of the retinal image (p. 979). 



19. Colour-Blindness. Spread out Holmgren's coloured wools on 

 a sheet of white filter-paper in a good light. Do not mention the 

 colours of any of the wools, but (i) ask the person who is being tested 

 to pick out all the wools which seem to him to match a pale pure green 

 wool (neither yellow green nor blue green), which is handed to him. 

 He is not to make an exact match, but to pick out the skeins which 



Fig. 460. Apparatus for Colour-Mixing. 



seem to have the same colour. If he makes any mistakes, by selecting, 

 e.g., in addition to the green skeins, any of the ' confusion colours/ 

 such as grey, greyish-yellow, or blue wools, there is some defect of 

 colour discrimination. To determine whether the person is red or green 

 blind, tests (2-) and (3) are then made. (2) Give him a medium purple 

 (magenta) wool, and ask him to pick out matches for it. If he is red- 

 blind, he will select as matches to it only blues and violets, as well as 

 other "purples. If he is green-blind, he will select only greens and 

 greys. (3) The third test is a red wool. In selecting matches for this, 

 the red-blind will choose (with reds) greens, greys, or browns less bright 



