SUCCESSIVE LIGHT INDUCTION. 287 



glass, and get the reflected image of the yellow to overlap 

 the blue, seen directly through the glass ; where they overlap 

 appears white. 



3. To Test Colour-Blindness. On 110 account is the person being 

 tested to be asked to name a colour. In a large class of students 

 one is pretty sure to find one or more who are more or less colour- 

 blind. The common defects are for red and green. 



(a.) Place Holmgren's worsteds on a white background in 

 a good light. Select, as a test colour, a skein of a green 

 colour, such as would be obtained by mixing a pure 

 green with white. Ask the examinee to select and pick 

 out from the heap all those skeins which appear to him 

 to be of the same colour, whether of lighter or darker 

 shades. A colour-blind person will select amongst others 

 some of the confusion-colours e.g., pink, yellow. A 

 coloured plate showing these should be hung up in the 

 laboratory. Any one who selects all the greens and no 

 confusion-colours, has normal colour Vision. If, however, 

 one or more confusion -colours be selected, proceed as 

 follows : Select, as a test colour, a skein of pale rose. If 

 the person be red-blind, he will choose blue and violet ; if 

 green-blind, grey and green. 



(6.) Select a bright red skein. The red-blind will select 

 green and brown ; the green-blind picks out reds or lighter 

 brown. 



4. Successive Light Induction. 



(a.) Look for one minute at a small white circular disc on 

 a black background e.g., velvet. Close and cover the eyes. 

 A negative after-image of the disc appears, but it is darker 

 and blacker than the visual area, and it has a peculiar light 

 area round it, brightest close to the disc, and fading away 

 from it. 



(6.) Look at two small white square patches of paper 

 placed one-eighth of an inch apart on a black background. 

 On closing the eyes, the black space between them looks 

 brighter than the other three sides of the squares. 



(c.) Look at a black strip on a white ground. On closing 



