Exercise XXVI 



SENSORY RECEPTORS 133 



Blind spot 



of the retina, so fixing its position on the retinal 

 surface.) The experimenter, without jogging the 

 paper, slowly advances a small target (a pencil 

 point will do) into the subject's field of vision, 

 starting about 2 to 4 inches to the right of the 

 cross. There is a point at which the target dis- 

 appears. The experimenter marks this point on 

 the paper, and starts again from another angle. 

 By repeating this performance, advancing the 

 target from various angles around the cross, one 

 can plot the entire boundary of the blind spot. 



If there is time, repeat this for the left eye; 

 this time, however, the target should be intro- 

 duced at the left of the cross. 



The accompanying figure shows diagram- 

 matically the optics of this experiment. Study 

 it carefully, and see that you understand every- 

 thing in it, for it contains the essential elements 

 of image formation in the eye. From the results 

 of your experiment calculate the diameter of 

 the blind spot in the eye, and also the distance 

 of its center from the fixation point within the 

 central fovea. This is done very easily, since 

 the projected dimensions on the paper are to 

 the dimensions on the retinal surface as the 

 distance from the paper to the eye (300 mm) 

 is to 17 mm, the focal length of the human eye. 



Note that, like any other simple lens system, 

 the eye inverts the images of all objects at which 

 one looks, and equally inverts all spatial rela- 

 tions. Why then do we not see upside down, 

 and wrong end to? 



Retinal blood vessels 



As a consequence of the way it develops 

 embryologically, the vertebrate retina points 

 away from the light. Light must pass through 

 the entire thickness of the retina, including the 

 retinal blood vessels, before reaching the visual 

 receptors. The blood vessels therefore cast a 

 continuous shadow upon the visual field; and 

 the only reason we are not aware of this at all 

 times is that one cannot continue to see any 

 image that is fixed in position on the retinal 

 surface. To make the blood vessels visible, all 

 that is needed is to make their shadows move. 



Make a small hole, about 1 mm across, in a 

 card, and look through it at a brightly illumi- 

 nated white surface, meanwhile giving the hole 

 a rapid side-to-side or rotary motion. Shortly 

 you should become aware of a delicate, lacy 

 network, with a central open space, as though 

 a hole were torn in it. The hole will move 

 wherever the eye is fixated. The network repre- 

 sents the shadows of the retinal capillaries, the 

 hole the central fovea, from which blood vessels 

 are lacking. 



Those of you who see this plainly might like 

 to estimate the diameter of the fovea. This can 

 be done by estimating the width of the image of 

 the capillary-free area as projected on the white 

 surface; from this and the distance of the sur- 

 face from the eye, you can complete the calcu- 

 lation as you did above for the blind spot. 



