174 LETTERS ON NATURAL MAGIC. 



protuberant parts of objects appeared 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. Gme- 

 lin is said to have constantly observed the following 

 effects: Whenever he viewed any object rising upon a 

 plane of any colour whatever, 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 perpendicularly, 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 object 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 dis- 

 covered a method of preventing 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 

 in the whole. " But why these things should so happen, 

 he did not pretend to determine." 



The best method of observing this deception is to view 

 the engraved seal of a watch with the eye-piece of an 

 achromatic telescope, or with a compound microscope, or 

 any combination of lenses which inverts the objects that 

 are viewed through it.* The depression in the seal will 



* A single convex lens will answer the purpose, provided we hold 

 the eye six or eight inches behind the image of the seal formed in 

 is conjugate focus. 



