296 THE SPECIAL SENSES 



energy " of the optic nerve, which nerve, in common with the 

 other nerves of the special sense, obeys a general law of nature, 

 which requires that, whenever one of these nerves is stimulated, 

 it shall respond with the sensation peculiar to itself. These 

 flashes of retinal light have no power to illuminate external 

 objects, although the opposite of this statement has been main- 

 tained. On the occasion of a remarkable trial in Germany, it 

 was claimed by a person who had been severely assaulted on a 

 very dark night, that the flashes of light caused by repeated 

 blows upon the head enabled him to see with sufficient distinct- 

 ness to recognize his assailant. But the evidence of scientific 

 men entirely refuted this claim, by pronouncing that the eye, 

 under the circumstances named, was incapacitated for vision. 

 Too intense light occasions a feeling of pain, but it is of a 

 peculiar kind, and is termed " dazzling." 



69. All parts of the retina are not equally sensitive, and 

 singularly enough, the point of entry of the nerve of sight in 

 the back part of the eyeball is entirely insensible to light, and 

 is called the " blind spot." The existence of this point r may 

 be proved by a simple experiment. Hold the accompanying 

 figure, on page 283, directly in front and parallel with the eyes. 



FIG. 66 



Close the left eye, and fix the sight steadily on the left-hand 

 circle ; then, by gradually varying the distance of the figure 

 from the eye, at a certain distance (about six inches), the right- 

 ,hand circle will disappear, but nearer or further than that, it 

 will be plainly seen. The other eye may be also tried, with a 

 similar result : if the gaze be directed to the right-hand circle, 

 the left one will seem to disappear. The experiment may be 



69. Sensitiveness of all parts of the retina ? Experiment to prove the existence of the 

 "blind spot"? 



