104 LETTERS ON NATURAL MAGIC. 



tinge which is seen when the eyelids are closed, and 

 which is produced by the light which passes through 

 them. The luminous circles too always continue while 

 the pressure is applied, and they may be produced as 

 readily after the eye has been long in darkness as when it 

 has been recently exposed to light. When the pressure 

 is very gently applied, so as to compress the fine pulpy 

 substance of the retina, light is immediately created 

 when the eye is in total darkness ; and when in this state 

 light is allowed to fall upon it, the part compressed is 

 more sensible to light than any other part, and conse- 

 quently appears more luminous. If we increase the 

 pressure, the eyeball, being filled with incompressible 

 fluids, 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 con- 

 clusions, that when the retina is compressed in total 

 darkness it gives out light ; that when it is compressed 

 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 im- 

 pressions. 



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 receives a sudden blow, a 



