126 ELEMENTARY PHYSIOLOGY. XXIII. 



and between it and the paper place a book edgeways, or any object 

 which will throw a shadow on the paper. Between the paper and the 

 window place a similar object, throwing a like shadow. The distance 

 of the candle should be such that the two shadows are of nearly 

 equal intensity. 



The shadow from the candle, though illuminated by the white 

 sunlight, will appear blue, the complement of the reddish-yellow 

 colour of the rest of the paper illuminated by the candle. 



Through a small black tube, e.g. a piece of black paper rolled up, 

 look at a point on the edge of the blue shadow so that half the field 

 of view is blue and the other white (or yellowish). While looking 

 let someone blow out the candle; the half of the field previously 

 blue will now become faintly yellow, and the white (or yellowish) 

 half will become blue. 



The daylight -shadow heightens the effect on the candle shadow, 

 but may be dispensed with. 



In place of sunlight and candle, two coloured lights may be used. 



In the above experiments avoid looking at the colours too fixedly 

 and for too long a time. Otherwise the icsults will be modified by 

 after-images. 



20. Test for red-green blindness, by Holmgren's 

 wools. All the wools are spread out on the table. B (who 

 has been found to have normal colour-vision) gives the 

 light green wool labelled No. 1 to A. A selects 1 six 

 wools of the same colour as No. 1, but of different shades 

 (saturation). A, if he is red-green colour-blind, will 

 probably select one or more of the red shades. This is 

 tested further by B giving A the light red wool labelled 

 No. 2 to match. 



Yellow-blue colour-blindness: a similar experiment is made, match- 

 ing light yellow and light blue. 



1 Note should be made of the wools taken up by A compared with the 

 test wool and then rejected, since a slightly colour-blind person will 

 compare wools which would not be compared by a person with normal 

 vision. 



