90 Optical Illusion in Examining a Dioramic Picture 



the lower end of the pin comes from the upper part of the 

 wall or window E, the shadow must necessarily appear invert- 

 ed with respect to the object. 



The following variation of Le Cat's experiment has been de- 

 scribed by the writer of this article. Take a common pin and 

 hold it in any position near the eye, so that the observer sees re- 

 flected from its head a faint circle of light, then hold a second 

 pin opposite to it exactly as in Fig. 9, and an inverted image of 

 the one pin will be seen in the head of the other. If the head 

 of the first pin is round and well polished, the inverted and 

 magnified image of the other will be more distinct. In this 

 form of the experiment a diverging pencil of light from the 

 window or a candle replaces the diverging pencil in Fig. 15, 

 which proceeds from the perforation in the card CB, and of 

 course produces the same effect. The little round knob, by 

 the pressure upon which the case of a watch is often opened, 

 will answer better than the finest pin head. 



No. IX. Optical Illusion in examining a Dioramic Picture. 



In examining a dioramic representation of the inside of Ro- 

 chester cathedral, which produced the finest effect from the 

 entire exclusion of all extraneous light, and of all objects, ex- 

 cepting those on the picture itself, the writer of this article 

 was struck with an appearance of distortion in the perspective, 

 which he ascribed to the canvas not hanging vertically. Upon 

 mentioning this to the gentleman who exhibited the picture, he 

 offered to walk in front of the picture, and strike its surface with 

 the palm of his hand, to show that the canvas was freely sus- 

 pended. Upon doing this a very remarkable deception took 

 place. As his hand passed along, it gradually became larger 

 and larger, till it reached the middle, when it became enor- 

 mously large. It then diminished till it reached the other end 

 of the canvas. 



As the hand moved towards th«^ middle of the picture, it 

 touched parts of the picture more and more remote from the 

 eye of the observer, and consequently the mind referred the 

 hand and the object in contact with it to the same remote dis- 

 tance, and consequently gave it an apparent magnitude such 



