LESSON XLII 



THE SENSE ORGANS (Concluded) 

 VISION 



1. The Blind Spot. Make a small cross mark upon a white strip of 

 paper. About 8 cm. to the right of it make a black dot of the size of 

 a large pea. Close your left eye and with your right eye gaze fixedly 

 at the cross lines, allowing the dot to lie in the outer visual field. Bring 

 the card closer to the eye until the dot disappears completely. Move 

 the card still closer; the dot reappears. Draw a diagram explaining 

 this defect in the visual field. What position does the yellow spot 

 occupy in relation to the blind spot, and how is this disturbance over- 

 come in binocular vision. 



With your right eye gaze at an object placed at a distance of 6 m. 

 from you. Adjust the position of a fellow student in such a way that 

 his head disappears completely. 



Make a small cross upon a white strip of paper. On each side of it 

 and about 4 cm. from it make two black dots. Hold your left hand 

 with its inner margin against the bridge of your nose. With your right 

 hand hold the above figure in front of your eyes, and while steadily 

 fixing the cross move the paper to and fro until you reach the distance 

 at which both dots disappear. 



2. The Contours of the Blind Spot. Place the chin of the subject 

 upon a support and adjust a sheet of white paper about 50 cm. vertically 

 in front of him. Ask him to gaze with his right eye at a small black 

 dot upon the paper. Fasten a pin with a large black head upon a straw, 

 and move this pin from without inward along the horizontal meridian 

 of the eye. Indicate upon the paper the moments when the head of the 

 pin disappears and reappears. Draw a vertical line about midway be- 

 tween these two points and also several oblique lines. Indicate upon 

 all these lines the moments when the pin disappears and reappears. 

 Connect these points to obtain a continuous line. Note that the field 

 is irregularly oval in its outline, owing to the fact that the blood-vessels 

 emerging from the optic disk are also insensitive to the light rays. 



3. The Yellow Spot. Having rested the eye for a minute or two, 

 look through a flat bottle containing a fairly strong solution of chrome 

 alum. It is best to hold the bottle against a sheet of white paper. 

 Since the pigment of the yellow spot absorbs the blue and green rays 

 and transmits the others, the predominant tinge imparted to the area 

 corresponding to the macula lutea will be red (purple). 



4. The Retinal Blood-vessels. While the subject turns his eyes 

 laterally upon a dark wall, concentrate a beam of light from the optical 

 lantern upon the exposed sclerotic coat directly behind the region of 



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