3 $4 PRACTICAL PHYSIOLOGY. [LXXIV. 



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

 good light. Select, as a test colour, a skein of a light 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 labora- 

 tory. 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 chose blue 

 and violet ; if green-blind, grey and green. 



(/;.) Select a bright red skein. The red-blind will select green 

 and brown : the green-blind picks out reds or lighter brown. 



5. Contrast and Simultaneous Contrast. 



The following are examples of simultaneous contrast where 

 stimulation of the retino-cerebral apparatus modifies the sensations 

 excited by a different portion of the retina when the compared 

 objects light or colour are looked at simultaneously. Contrast 

 phenomena were carefully studied by Chevreul in relation to the 

 effects produced by colours juxtaposed in tapestry in the Gobelin's 

 factory of Paris. Contrast may apply to size, light, colour, and it 

 may be simultaneous or successive. 



(a.) Place a small white square or oblong piece of paper or cross 

 on a dull, black surface. Stare steadily at the white square, and 

 observe that the edges appear whiter than the centre ; indeed, the 

 centre by contrast may appear greyish. A white strip of paper 

 placed between two black strips, looks white at the margin near 

 the black. 



(b.) Look with one eye at the sky through a i-inch blackened 

 tube, both eyes being open. The field of vision looks much brighter 

 when seen through the tube than is the case with the other eye. 



(f.) Place side by side a white and black surface. Cut two 

 oblong (ii"x J") pieces of grey, yellow, or other coloured paper 

 of exactly the same size, and lay one piece of the grey on the white 

 background, and the other on the black. Observe how much 

 brighter the latter looks owing to contrast. Eeverse the pieces, 

 and notice that the same result occurs. Repeat with other 

 colours. 



(d.) On the rotating machine cause a disc, as in fig. 276, to 

 rotate with moderate rapidity, when several zones will be seen, the 

 innermost black, while each one farther outwards is lighter in tint. 

 Each zone, where it abuts against the inner darker zone, is lighter 



