134 LETTERS ON NATURAL MAGIC. 



illusions, we should discover many circumstances favour- 

 able to these views. In those seen by Nicolai the in- 

 dividual figures were always somewhat paler than natural 

 objects. They sometimes grew more and more indistinct, 

 and became perfectly white ; and, to use his own words, 

 " he could always distinguish, with the greatest precision, 

 phantasms from phenomena." Nicolai sometimes saw the 

 spectres when his eyes were shut, and sometime^ they 

 were thus made to disappear, effects perfectly identical 

 with those which arise from the impressions of very 

 luminous objects. Sometimes the figures vanished en- 

 tirely, and at other times only pieces of them disappeared, 

 exactly conformable to what takes place with objects seen 

 by indirect vision, which most of those figures must 

 necessarily have been. 



Among the peculiarities of spectral illusions there is 

 one which merits particular attention, namely, that they 

 seem to cover or conceal objects immediately beyond them. 

 It is this circumstance more than any other which gives 

 them the character of reality, and at first sight it seems 

 difficult of explanation. The distinctness of any impres- 

 sion on the retina is entirely independent of the accommo- 

 dation of the eye to the distinct vision of external objects. 

 When the eye is at rest, and is not accommodated to 

 objects at any particular distance, it is in a state for 

 seeing distant objects most perfectly. When a distinct 

 spectral impression, therefore, is before it, all other objects 

 in its vicinity will be seen indistinctly, for while the eye 

 is engrossed with the vision, it is not likely to accommo- 

 date itself to any other object in the same direction. It 

 is quite common, too, for the eye to see only one of two 

 objects actually presented to it. A sportsman who has 

 been in the practice of shooting with both his eyes open 

 actually sees a double image of the muzzle of his fowling- 

 piece, though it is only with one of these images that he 



