122 VERY BKIGHT LIGHTS. 



You must understand, however, that, as was explained 

 in describing such spectral illusions, nothing of all these 

 arrangements and effects is seen by the spectators in front, 

 except the ultimate image seen in the upper glass, and ap- 

 pearing as if it stood upon the stage. The plates of glass j 

 the course of the rays, from the source of light in the in- 

 strument where the man is sitting, through its zigzag 

 course to the eyes of the spectators ; the two images, one 

 on the lower and one on the upper glass, as well as the 

 form and position of the glasses themselves, are all shown 

 in the engraving of this double reflection for the purpose 

 of showing what the actual track of the rays through the 

 air is in such a case. But we never really see rays passing 

 thus through the air before us. The eye takes no cognizance 

 of any rays except those which actually enter it, and are 

 concentrated by the lens into an image upon the -retina. 

 Thus the spectators, in the case of an illusion like this, 

 would be wholly unconscious of all these movements of the 

 light, and even of the existence of the glasses, although one 

 of them would be full before them. The light would only 

 enter their eyes as it was reflected the last time, which 

 would be exactly as if it came from a figure standing behind 

 the glass upon the stage, and thus the illusion is created. 



In other words, the ghost seen is simply the reflection 

 of a real figure in a mirror. In ordinary cases we know 

 that the reflection seen in a mirror is an illusion, for the 

 mirror is silvered on the back so as to allow no light from 

 any thing really behind it to pass through, and thus only 

 the objects that are reflected in it can be seen. We see the 

 frame of it, moreover, so that we know that the mirror is 

 there. But, in the case of these spectres, the plates of 

 glass have no frames, and the edges of them are concealed, 

 and we see, moreover, objects through the glass as well as 

 those reflected in it. In ordinary cases, when we see ob 



