PHOSPHORESCENCE OF THE RETINA. 103 



reflecting light, it may suddenly disappear in a position 

 fully before, and within the reach of, the observer's eye ; 

 and if this evanescence takes place in a path or road 

 where there was no side-way by which the figure could 

 escape, it is not easy for an ordinary mind to efface the 

 impression which it cannot fail to receive. Under such 

 circumstances, we never think of distrusting an organ 

 which we have never found to deceive us ; and the truth 

 of the maxim that " seeing is believing," is too universally 

 admitted, and too deeply rooted in our nature to admit on 

 any occasion of a single exception. 



In these observations we have supposed that the spec- 

 tator bears along with him no fears or prejudices, and is 

 a faithful interpreter of the phenomena presented to his 

 senses ; but if he is himself a believer in apparitions, and 

 unwilling to receive an ocular demonstration of their 

 reality, it is not difficult to conceive the picture which 

 will be drawn when external objects are distorted and 

 caricatured by the imperfect indications 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 eyeball outwards by applying the point of the 

 finger between it and the nose, a circle of light will be 

 seen, which Sir Isaac Newton describes as " a circle of 

 colours like those in the feather of a peacock s tail." He 

 adds, that " if the eye and the finger 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 



