18 LETTEES ON NATURAL MAGIC. 



cations of his senses, and coloured with all the 

 vivid hues of the imagination. 



Another class of ocular deceptions have their 

 origin in a property of the eye which has been 

 very imperfectly examined. The fine nervous 

 fabric which constitutes the retina, and which 

 extends to the brain, has the singular property 

 of being phosphorescent by pressure. When we 

 press the eye-ball outwards by applying the point 

 of the finger between it and the nose, a circle of 

 light will be seen, which Sir Isaac Newton de- 

 scribes as " a circle of colours like those in the 

 feather of a peacock's tail." He adds, that " if 

 the eye and the figure remain quiet, these colours 

 vanish in a second of time ; but if the finger be 

 moved with a quavering motion, they appear 

 again." In the numerous observations which I 

 have made on these luminous circles, I have 

 never been able to observe any colour but white, 

 with the exception of a general red 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 con- 

 tinue 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 consequently appears 

 more luminous. If we increase the pressure, the 

 eye-ball, being filled with incompressible fluids, 



