LXXIV.] CONTRAST. 359 



red light of a caudle, and illuminated by the daylight, appears blue, i.e., a 

 purely subjective blue, the complementary 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 i8 inches look at the centre of 

 the blue shadow, and let an observer cut off the light from thi^ 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 background just beyond it, the blue appears. 



{h.) In a window-shutter of a dark room cut two square holes (lo cm.) on 

 the same horizontal plane, and 2 feet apart. In one fix a piece of clear glass 

 to admit ordinary white light, and into the other fit a red or green coloured 

 glass. Both openings must be provided with a movable shutter to regulate 

 the amount of light admitted. At 3-4 feet distance place a rod or flat piece 

 of wood vertically against a white surface. Observe two shadows. Suppose 

 the glass to be red, then the shadow due to the ordinary light is red, that of 

 the red glass is greenish. Substitute for the red light that of a lighted candle. 

 The shadow then appears blue. 



10. Choroidal Illumination. 



(a.) In a dark room light an ordinary lamp or fan-tailed gas-burner. Place 

 the source of light at the right side, about 2 feet from an open book or sheet 

 of paper. Partly separate the fingers of the left hand and place them over the 

 face, so that different portions of the paper are seen by each eye. That half of 

 the page seen with the right eye has a greenish tint, the other part seen 

 with the left eye is red or pinkish. Change the source of light to the left side, 

 the colours are reversed. 



{b.) With the conditions as in (a.), hold a piece of paper (3-4 cm. wide), ora 

 visiting-card, between the eyes with its flat surface towards the face, the same 

 phenomena are seen. 



(c.) Cut in a piece of black cardboard two rectangular holes (4 x 10 mm.), 

 separated by a di.stance about equal to that between the pupils, with the con- 

 ditions as in {a.). Hold the cardboard about 10 inches or more from you, 

 and look through the holes at a white surface ; four images of the two holes 

 will be seen ; the inner right and outer left images are impressions f^rom the 

 right eye, the inner left and outer right from the left eye. This is easily 

 proved by closing either eye, when the images belonging to that eye disappear. 

 If the source of light be on the right side, the former pair of images is greenish 

 in colour, the latter is pale pink. Change the light to the left side and the 

 colours are reversed (//. Sewall). Tlie colour- phenomena occur witliout the aid 

 of objective colour, and are due to light passing through the sclerotic and 

 choroid coats. 



11. Binocular Contrast. 



Place a white strip of paper on a black surface, look at the white paper 

 and squint so as to get a double image. In front of the right eye hold a blue 

 glass, and in front of the left one a grey (smoked) glass. The image of the 

 right eye will be blue, that of the left yellow. Instead of the grey glass, a 

 card with a small hole in it placed in front of the left eye does perfectly well. 

 The yellow of the left eye is a contrast sensation. 



12. Positive After-images. 



(a.) In a room, not too brightly illuminated, rest the retina by 

 closing the eyes for a minute. Suddenly look for two seconds at a 



