COLOURED SHADOWS. 291 



one might expect to be grey, from the black and white, 

 appears reddish i.e., the complementary colour of the 

 greenish ground. 



(h.) Place a strip of grey paper on a black background and 

 a corresponding strip on a white ground. The former will 

 -appear much lighter, the grey on white much darker. Fix 

 the eyes for a minute on a point midway between the 

 strips, close and cover the eyes. The after-images will show 

 a great difference in luminosity. 



(i.) Ragona Scina's Experiment. Two pieces of wood fixed 

 .at right angles to each other are covered by white paper, 

 while a coloured sheet of glass is held at an angle of 45 

 between them (Fig. 140). Look vertically through the glass 

 at the horizontal white paper, and observe 

 & pale red tint. Attach a small black 

 square to the centre of the vertical arm 

 at B, the image of this square is seen at 

 b as a deep red image. Place a similar 

 black square on the horizontal board at 

 C, it should appear grey; but a grey on 

 a red ground causes contrast, and so one 

 sees a greenish-blue square alongside a red Fig. 140. Ragona 



Son.. Expen- 



7. Coloured Shadows. 



(a.) Place an opaque vertical rod (1 inch in diam.) in front 

 of a white background. Admit not too bright daylight to 

 cast a shadow of the rod. Place a lighted candle behind 

 one side of the rod, the shadow caused by the yellow- 

 red light of a candle, and illuminated by the daylight, 

 appears blue i.e., a purely subjective blue, the complemen- 

 tary colour of the yellow-red light of the candle, which casts 

 a yellow light. The effect is more pronounced, the darker 

 both shadows are. To show that the blue is purely subjective, 

 roll up a sheet of black paper black surface innermost in 

 the form of a tube about ^ inch or less in diameter. At a 

 distance of 18 inches, look at the centre of the blue shadow, 

 and let an observer cut off the light from the candle by 

 means of an opaque screen. On removing the screen, no 

 change is visible, but if the tube be directed to the line of 

 junction of the blue shadow, with the illuminated back- 

 ground just beyond it, the blue appears. 



