ELEMENTARY EXPERIMENTAL PHYSIOLOGY 21 



from the part of the paper where it is invisible to where it becomes 

 visible. Mark the limits of the invisible area in all directions from 

 its centre. 



Examine the retina in the model eye, and note that the blind area 

 discovered corresponds in position with the entrance of the optic 

 nerve (p. 14, II.). 



3. Is the Power of Localising the Source of Light equally developed 

 all over the Retina? 



METHOD. Prepare an experiment as above, but instead of a 

 pencil take a pair of 

 compasses. Bring the 

 points close together, 

 and place them on the 

 central mark, and then 

 note whether both can 

 be seen. Now, keep- 

 ing the head perfectly 

 steady and the eye 

 fixed on the central 

 mark, draw them along 

 the paper away from 



ppntral mark and FlGl 14 '~~ To show the decreasin g power of locali- 



entrai m J-K, ana gation on passing from the centre to the peri . 



phery of the retina. 



(From Noel Paton's Essentials.) 



note when two points 

 can no longer be dis- 

 tinguished. 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. 



In a good section through the central spot of the retina it may be 

 noticed that, while at that point cones alone exist, further out upon 

 the retina these are situated at greater and greater distance with 

 rods between them. Apparently it is necessary that two cones 

 should be stimulated to get a double sensation. 



-^ 4. What Layer of the Retina is 



acted upon by Light ? 



METHOD. In a dark room 

 stand side on against a uni- 

 * formly coloured wall, with the 



eyes turned towards the wall. 

 By means of a lens another 



v , _ , f student directs a powerful ray of 



FIG. 15. To show how shadows of ,.,,,, , J , 



the blood vessels are thrown on ^t through the exposed scle- 



the layers of rods and cones in the rotic COat of the eye, and, on 



experiments of Purkinje's images, moving the light up and down, 



(From Noel Paton's Essentials.) an ^ f rom gide to si ^ any 



appearance on the wall is noted. 



The lines seen are shadows of the retinal blood vessels Purkinje's 

 Images. Revise your knowledge of the distribution of the blood 



