ELEMENTARY EXPERIMENTAL PHYSIOLOGY 27 



D. MODIFIED VISUAL PERCEPTIONS 



1. Effects of Strong or Prolonged Stimulation. 



METHODS. (a) After keeping the eyes closed for two minutes 

 look steadily at a white mark on a black surface for a few seconds 

 and then close the eyes. 



Describe the image that appears and also any change in its 

 intensity as time passes- Positive After-image. 



(6) Look steadily at the same mark for three minutes and then 

 close the eyes. 



Describe the image as above Negative After-image. 



2. What is the Effect of prolonged Stimulation of the Eye with any one 

 Part of the Spectrum ? 



METHOD. Put a disc of red paper upon a white ground in a 

 strong light. Look steadily at it for half a minute, then remove it 

 and continue to look at the white surface, and note what happens. 

 Repeat this with discs of different colours. 



The colour which appears is said to be complementary to the first. 



3. The Possibility of Erroneous Interpretation of Visual Sensations. 



1. Two squares of equal size are fixed upon paper : one is white 

 placed upon a black ground, and one black placed upon a white 

 ground. Which appears larger ? 



2. Place three equidistant dots in a straight line on a piece of 

 paper and subdivide one division by a series of dots. Which part 

 appears longer ? 



3. (a) A red square is placed on a white ground and another on 

 a green ground. Which appears redder in colour ? 



(6) A red paper is placed on the table with a grey one a foot away 

 on one side and a green one a similar distance away on the other. 



Does the red appear redder after looking at the grey or at the 

 green sheet. 



4. Make two marks on a sheet of paper as in Mariotte's Experi- 

 ment (p. 20, II. 2) but at about 1 \ to 2 inches apart, or draw a cage in 

 the position of one mark and a bird in the position of the other. With 

 both eyes open hold the paper near the near point of vision and 

 then focus the eyes for a distant object through the paper. Note 

 what seems to happen to the marks or to the bird and the cage. 

 Try to explain this. The rays from a distant object are practically 

 parallel and falling on the central spots of the two eyes give the 

 idea of a single object. When by this artifice the image of the bird 

 and of the cage fall upon the two central spots they are mentally 

 referred to the same place and seem to coincide. 



5. Rule a square with parallel diagonal lines, and place short 

 vertical and horizontal lines upon the alternate diagonals. Do the 

 latter now appear parallel ? 



From these experiments draw your conclusions as to the necessary 

 accuracy of the knowledge gained by vision. 



(Read Modified Visual Perception in Text Book.) 



