28 LETTEES ON NATUBAL MAGIC. 



As the revolving card is virtually transparent, 

 so that bodies beyond it can be seen through it, 

 the power of the illusion might be greatly ex- 

 tended by introducing into the picture other 

 figures, either animate or inanimate. The setting 

 sun, for example, might be introduced into a 

 landscape ; part of the flame of a fire might be 

 seen to issue from the crater of a volcano, and 

 cattle grazing in a field might make part of the 

 revolutionary landscape. For such purposes, 

 however, the form of the instrument would re- 

 quire to be completely changed, and the rotation 

 should be effected round a standing axis by 

 wheels and pinions, and a screen placed in front 

 of the revolving plane with open compartments 

 or apertures, through which the principal figures 

 would appear. Had the principle of this instru- 

 ment been known to the ancients, it would doubt- 

 less have formed a powerful engine of delusion 

 in their temples, and might have been more 

 effective than the optical means which they seem 

 to have employed for producing the apparitions 

 of their gods. 



In certain diseased conditions of the eye, effects 

 of a very remarkable kind are produced. The 

 faculty of seeing objects double is too common 

 to be noticed as remarkable ; and though it may 

 take place with only one eye, yet, as it generally 

 arises from a transient inability to direct the axis 

 of both eyes to the same point, it excites little 

 notice. That state of the eye, however, in which 

 we lose sight of half of every object at which we 

 look, is more alarming and more likely to be 

 ascribed to the disappearance of part of the object 

 than to a defect of sight. Dr. Wollaston, who 

 experienced this defect twice, informs us that, 



