14 LETTERS ON NATUEAL MAGIC. 



its effects, and important in its consequences, is 

 another illusion of the eye which presented itself 

 to me several years ago. When the eye is 

 steadily occupied in viewing any particular ob- 

 ject, or when it takes a fixed direction while the 

 mind is occupied with any engrossing topic of 

 speculation or of grief, it suddenly loses sight of, 

 or becomes blind to, objects seen indirectly, or 

 upon which it is not fully directed. This takes 

 place whether we use one or both eyes, and the 

 object which disappears will reappear without 

 any change in the position of the eye, while 

 other objects will vanish and revive in succession 

 without any apparent cause. If a sportsman, for 

 example, is watching with intense interest the 

 motions of one of his dogs, his companion, though 

 seen with perfect clearness by indirect vision, will 

 vanish, and the light of the heath or of the sky 

 will close in upon the spot which he occupied. 



In order to witness this illusion, put a little bit 

 of white paper on a green cloth, and, within three 

 or four inches of it, place a narrow strip of white 

 paper. At the distance of twelve or eighteen 

 inches, fix one eye steadily upon the little bit of 

 white paper, and in a short time a part or even 

 the whole of the strip of paper will vanish as if 

 it had been removed from the green cloth. It 

 will again reappear, and again vanish, the effect 

 depending greatly on the steadiness with which 

 the eye is kept fixed. This illusion takes place 

 when both the eyes are open, though it is easier 

 to observe it when one of them is closed. The 

 same thing happens when the object is luminous. 

 When a candle is thus seen by indirect vision, it 

 never wholly disappears, but it spreads itself out 



