292 EXPERIMENTAL PHYSIOLOGY. 



(b.) In a window-shutter of a dark room, cut two square 

 holes (10 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 to 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. 



8. 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), or a 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 (4x10 mm.), separated by a distance about equal 

 to that between the pupils, with the conditions 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 from 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 (H. Sewall). The colour-phenomena occur without 

 the aid of objective colour, and are due to light passing 

 through the sclerotic and choroid coats. 



