CONVERSION OF CAMEOS INTO INTAGLIOS. 99 



guinea through a compound microscope of new 

 construction, was surprised to see the head upon 

 the coin depressed, while other members could 

 only see it embossed as it really was. 



While using telescopes and compound micro- 

 scopes, Dr. Gmelin of Wurtemburg observed the 

 same fact. The protuberant parts of objects ap- 

 peared to him depressed, and the depressed parts 

 protuberant : but what perplexed him extremely, 

 this illusion took place at some times and not at 

 others, in some experiments and not in others, 

 and appeared to some eyes and not to others. 



After making a great number of experiments, 

 Dr. Gmelin is said to have constantly observed 

 the following effects : Whenever he viewed any 

 object rising upon a plane of any colour what- 

 ever, provided it was neither white nor shining, 

 and provided the eye and the optical tube were 

 directly opposite to it, the elevated parts appeared 

 depressed, and the depressed parts elevated. This 

 happened when he was viewing a seal, and as 

 often as he held the tube of the telescope perpen- 

 dicularly, and applied it in such a manner that its 

 whole surface almost covered the last glass of the 

 tube. The same effect was produced when a 

 compound microscope was used. When the ob- 

 ject hung perpendicularly, from a plane, and the 

 tube was supported horizontally and directly 

 opposite to it, the illusion also took place, and the 

 appearance was not altered when the object hung 

 obliquely and even horizontally. Dr. Gmelin is 

 said to have at last discovered a method of pre- 

 venting this illusion, which was by looking, not 

 towards the centre of the convexity, but at first 

 to the edges of it only, and then gradually taking 

 H 2 



