GENERAL VIEWS. 135 



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 one alone visible, and by the same process 

 the red one may be made to disappear. In this case, 

 however, the two patterns, like the two images, may be 

 seen together ; but if the very same portion of the retina 

 is excited 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 fundamental sounds. It is quite pos- 

 sible, however, that the brightest parts of a spectral figure 

 may be distinctly seen along with the brightest parts of 

 an object immediately behind it, but then the bright parts 

 of each object will fall upon different parts of the retina. 



These views are illustrated by a case mentioned by 

 Dr. Abercrombie. A. gentleman, who was a patient of 

 his, of an irritable habit, and liable to a variety of uneasy 

 sensations in his head, was sitting alone in his dining- 

 room in the twilight, when the door of the room was a 

 little open. He saw distinctly a female figure enter, 

 wrapped in a mantle, with the face concealed by a large 

 black bonnet. She seemed to advance a few steps towards 

 him, and then stop. He had a full conviction that the 

 figure was an illusion of vision, and he amused himself 

 for some time by watching it ; at the same time observing 

 that he could see through the figure so as to perceive the 

 lock of the door, and other objects behind it.* 



* Inquiries concerning the Intellectual Powers, and the Investi- 

 gation of Truth. 



