16 LETTERS ON NATURAL MAGIC. 







Availing himself of the slightest difference of tint 

 in the adjacent heath, he keeps his eye steadily 

 fixed on it as he advances, but whenever the 

 contrast of illumination is feeble, he will invari- 

 ably lose sight of his mark, 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 per- 

 sons 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 dark- 

 ness. 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 accommodate itself to 

 see near objects distinctly, so that the forms of 

 persons and things actually become more sha- 

 dowy 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 frequent 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 

 white, because no other colour can be seen, and 

 they are either formed out of inanimate 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 de- 

 grees of light, its brighter parts may enable the 



