38 



directs a powerful ray of light through the exposed sclerotic 

 coat of the eye, and, on moving the light up and down, 

 and from side to side, any appearance on the wall is noted. 

 The lines seen are shadows of the retinal blood vessels. 

 Revise your knowledge of the distribution of the blood vessels 

 of the retina. CONCLUSION ? Is it the front layer of retina 

 or a back layer which is acted upon by light ? 



IV. Is the Power of Localising the Source of Light equally 

 developed all over the Retina ? 



METHOD. Prepare an experiment as on p. 37, II. 2, but 

 instead of a pencil take a pair of compasses. Bring the points 

 close together, and place the points on the central mark, and 

 note whether both can be seen. Now, keeping the eye fixed 

 on the central mark, draw them along the paper away from 

 the central mark, and note when two points can no longer be 

 distinguished. Separate them till they are again seen as two 

 and draw them still further out, and note what happens. 

 Record the result on a diagram and formulate your conclusions. 



V. What Range of Objects can be seen at one Time ? 

 The Field of Vision. 



METHOD. A. Black and White. Describe a semicircle on 

 a blackboard with the free ends of the line finishing at one 

 side. Mark the centre of the circle at the edge of the board 

 and the middle point in the circumference with an X. This 

 forms a rough " Perimeter." 



Keeping one eye closed, the observer places his other eye at 

 the centre and directs it steadily towards the middle point 

 in the circumference. A fellow student slowly draws a piece 

 of chalk along the circumference from below. The observer 

 notes when it becomes visible, and the other marks this point. 



Now, starting from above, the experimenter again draws 

 the chalk along the circumference until it becomes visible, 

 and marks the point where it conies into view. The angle 

 thus formed with the centre of the circle and these points 

 -subtends the vertical field of vision. Measure this angle and 

 record it. 



