THE SENSE ORGANS 203 



sheet of paper and cover the whole with tissue paper. Note the ap- 

 parent color of the strip. 



15. Irradiation. Place a circular piece of black paper 1 cm. in 

 diameter upon a white circle of paper 2 cm. in diameter, and a white 

 circle 1 cm. in diameter upon a black circle 2 cm. in diameter. Hold 

 the circles at some distance from the eyes and note their apparent size. 



Divide a square into four small squares of equal size, and blacken the 

 left upper and the right lower of these four fields. Observe that the 

 white squares appear to be larger than the black. 



16. Single and Double Images. Adjust your vision so as to see 

 clearly with both eyes a vertical rod held 60 cm. in front of them. 

 Hold upright the index-finger of one hand in the binocular line about 

 35 cm. from the eyes, and observe the double image of the finger. 

 Close the right eye and note whether the right or the left image dis- 

 appears. Accommodate for the finger, and observe the resulting double 

 image of the rod. Close one eye and note which image disappears. 

 Explain. 



Accommodate for an object placed about 2 m. in front of the eyes. 

 Hold a pencil upright in the binocular line about 30 cm. from the eyes. 

 Move the pencil slightly toward the right eye. Observe that one image 

 disappears when the images lie asymmetrically to one side of the line 

 of vision only, the image in the right eye being perceptible in the 

 present case. Close the right eye and in this way render the second 

 image perceptible. 



17. Binocular Fusion of Dissimilar Images. Fasten a red and a 

 green postage stamp upon a piece of cardboard at a distance equaling 

 the interocular distance. Observe them in the stereoscope and note 

 that their images are fused into a single one. 



18. Relation of Binocular Vision to Judgments of Direction. Draw 

 a sheet of paper, with a pinhole in it, horizontally past the eyes, starting 

 well to the right of the eye. To what plane of the head are the two 

 successive images referred? 



19. Relation of Binocular Vision to Judgments of Solidity. Ex- 

 amine a series of pictures in the stereoscope. Draw a diagram to show 

 the manner in which judgments of solidity are formed. 



20. Relation of Binocular Vision to Judgment of Distance. Close 

 one eye and hold the index-finger of the left hand vertically in front 

 of the other eye. Try to strike the left index-finger with the index- 

 finger of the right hand. 



Close one eye and try to dip the index-finger of one hand into the 

 mouth of a bottle held about 25 cm. in front of the other eye. 



With the left hand hold a pencil vertically about 25 cm. in front of 

 the eyes. Gaze at the pencil for a few seconds, close the left eye; then 

 cover the lower part of the pencil with your right index-finger held 

 vertically between the object and the eyes. Try to strike the pencil 

 with the finger. 



21. Influence of Convergence of the Visual Axes. Gaze at an 



