102 LETTERS ON NATURAL MAGIC. 



and if the retina is capable of taking it up, it is only to 

 lose it a second time. 



This illusion is likely to be most efficacious in the 

 dark, when there is just sufficient light to render white 

 objects faintly visible, and to persons who are either timid 

 or credulous must prove a frequent source of alarm. Its 

 influence too is greatly aided by another condition of the 

 eye, into which it is thrown during partial darkness. 

 The pupil expands nearly to the whole width of the iris 

 in order to collect the feeble light which prevails ; but it 

 is demonstrable that in this state the eye cannot accom- 

 modate itself to see near objects distinctly, so that the 

 forms of persons and things actually become more shadowy 

 and confused when they come within the very distance 

 at which we count upon obtaining the best view of them. 

 These affections of the eye are, we are persuaded, very fre- 

 quent causes of a particular class of apparitions which are 

 seen at night by the young and the ignorant. The spectres 

 which are conjured up are always ivhite, because no other 

 colour can be seen, and they are either formed out of inani- 

 mate objects which reflect more light than others around 

 them, or of animals or human beings whose colour or change 

 of place renders them more visible in the dark. When 

 the eye dimly descries an inanimate object whose different 

 parts reflect different degrees of light, its brighter parts 

 may enable the spectator to keep up a continued view of 

 it ; but the disappearance and reappearance of its fainter 

 parts, and the change of shape which ensues, will ne- 

 cessarily give it the semblance of a living form, and if it 

 occupies a position which is unapproachable, and where 

 animate objects cannot find their way, the mind will soon 

 transfer to it a supernatural existence. In like manner a 

 human figure shadowed forth in a feeble twilight may 

 undergo similar changes, and after being distinctly seen 

 while it is in a situation favourable for receiving and 



