54 LETTERS ON NATURAL MAGIC. 



pendent of the accommodation 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 indis- 

 tinctly, for while the eye is engrossed with the 

 vision, it is not likely to accommodate 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 sports- 

 man 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 covers 

 his game, having no perception whatever of the 

 other. But there is still another principle upon 

 which only one of two objects may be seen at a 

 time. If we look very steadily and continuously 

 at a double pattern, such as those on a carpet 

 composed of two single patterns of different 

 colours, suppose red and yellow; and if we direct 

 the mind particularly to the contemplation of the 

 red one, the green pattern will sometimes vanish 

 entirely, leaving the red alone visible ; and by 

 the same process the red one may be made to 

 disappear. In this case, however, the two pat- 

 terns, like the two images, may be seen together ; 

 but if the very same portion of the retina is ex- 

 cited by the direct rays of an external object, 

 when it is excited by a mental impression, it can 

 no more see them both at the same time, than a 

 vibrating string can give out two different fun- 

 damental sounds. It is quite possible, however, 

 that the brightest parts of a spectral figure may 



