108 LETTERS ON NATURAL MAGIC. 



glass or water, so as to moderate its brilliancy, and keep 

 the eye upon it steadily for a few seconds, we shall see 

 even for hours afterwards, and whether the eye is open 

 or shut, a spectre of the sun varying in its colours. At 

 first, with the eye open, it is brownish-red with a sky-l)lue 

 border, and when the eye is shut, it is green with a red 

 border. The red becomes more brilliant, and the Uue 

 more vivid, till the impression is gradually worn off; but 

 even when they become very faint, they may be revived 

 by a gentle pressure on the eyeball. 



Some eyes are more susceptible than others of these 

 spectral impressions, and Mr. Boyle mentions an indi- 

 vidual who continued for years to see the spectre of the 

 sun when he looked upon bright objects. This fact 

 appeared to Locke so interesting and inexplicable, that he 

 consulted Sir Isaac Newton respecting its cause, and 

 drew from him the following interesting account of a 

 similar effect upon himself : " The observation you mention 

 in Mr. Boyle's book of colours, I once made upon myself 

 with the hazard of my eyes. The manner was this : I 

 looked a very little while upon the sun in the looking- 

 glass with my right eye, and then turned my eyes into a 

 dark corner of my chamber, and winked, to observe the 

 impression made, and the circles of colours which encom- 

 passed it, and how they decayed by degrees, and at last 

 vanished. This I repeated a second and a third time. 

 At the third time, when the phantasm of light and 

 colours about it were almost vanished, intending my 

 fancy upon them to see their last appearance, I found, to 

 my amazement, that they began to return, and by little 

 and little to become as lively and vivid as when I had 

 newly looked upon the sun. But when I ceased to intend 

 my fancy upon them they vanished again. After this, I 

 found that, as often as I went into the dark, and intended 

 my mind upon them, as when a man looks earnestly to 



