THE SPECIAL SENSES. 207 



" specific energy" of the optic nerve, which nerve, in com- 

 mon with the other nerves of 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 maintained. On the occasion 

 of a remarkable trial in Germany, it was claimed by a per- 

 son 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 distinctness 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 foi 

 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 

 may be proven by a simple experiment. Hold the accom- 



FIG. 49. 



panying figure, on page 207, directly in front of and paral- 

 lel with the eyes. 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 dis- 

 tance (about six inches), the right-hand circle will disappear, 



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

 ence of the "blind spot. 1 ' 



