218 



PHYSIOLOGY FOR BEGINNERS 



CHAP'. 



blindness of this spot can easily be shown by throwing an 

 image on it. Hold the page you are reading about 1 2 inches 

 away, or nearer if you are short-sighted, close the left eye and 



look steadily with the right at the cross ; the large dot will be 

 seen as well. Now move the book slowly nearer, keeping the 

 eye fixed on the cross ; at a certain distance of the book the 

 dot will disappear, but will reappear as the book is moved 

 still closer. A, B, and C in Fig. 102 

 give the three positions. In all, the 

 image of the cross falls on the yellow 

 spot ; the image of the dot falls in A 

 between the yellow spot and the blind 

 spot, in B on the blind spot itself, and in 

 C on the other side of the blind spot 

 near the nose. 



Visual Sensations. The impres- 

 sion made by a flash of light on the 

 retina lasts a certain time after the light 

 is over. This is found to be about one- 

 eighth of a second. So that if two 

 flashes follow each other at a less inter- 

 val of time than this their impressions 

 catch each other up and produce one 

 sensation only. This is why the spokes 

 of a rotating wheel are not seen sepa- 

 rately, and why a lighted stick twirled 

 round gives the idea of a circle of 

 fire. 



The sensibility of the retina is easily 

 tired. If a bright light is looked at for 

 some time, and then the eye turned to 

 a sheet of paper, a dark spot or dark 

 image of the bright light is seen on the 

 paper. This is because the light from 

 that part of the paper is falling on a part of the retina which has 

 been so fatigued that the rays fail to excite the sensation of light. 

 Ordinary light, or white light as it is called, can be split up 

 into a number of rays which, falling on the retina, produce the 



FIG. 102. 



