EFFECTS OF PRESSURE ON THE RETINA. 19 



will protrude all round the point of pressure, and 

 consequently the retina at the protruded part will 

 be compressed by the outward pressure of the 

 contained fluid, while the retina on each side, 

 namely, under the point of pressure and beyond 

 the protruded part, will be drawn towards the 

 protruded part or dilated. Hence the part under 

 the finger which was originally compressed is 

 now dilated, the adjacent parts compressed, and 

 the more remote parts immediately without this 

 dilated also. Now we have observed, that when 

 the eye is, under these circumstances, exposed to 

 light, there is a bright luminous circle shading 

 off externally and internally into total darkness. 

 We are led, therefore, to the important conclu- 

 sions, that when the retina is compressed in total 

 darkness it gives out light ; that when it is com- 

 pressed when exposed to light, its sensibility to 

 light is increased; and that when it is dilated 

 under exposure to light, it becomes absolutely blind, 

 or insensible to all luminous impressions. 



When the body is in a state of perfect health, 

 this phosphorescence of the eye shows itself on 

 many occasions. When the eye or the head re- 

 ceives a sudden blow, a bright flash of light 

 shoots from the eyeball. In the act of sneezing, 

 gleams of light are emitted from each eye both 

 during the inhalation of the air, and during its 

 subsequent protrusion, and in blowing air vio- 

 lently through the nostrils, two patches of light 

 appear above the axis of the eye and in front of 

 it, while other two luminous spots unite into one, 

 and appear as it were about the point of the nose 

 when the eyes are directed to it. When we turn 

 the eyeball by the action of its own muscles, the 

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