100 On the Optical Illusion of' tin 



tached to them another kind of interest, not less deserving of 

 attention. To those who are in the practice of exercising a 

 presumptuous confidence in their own judgments, and who 

 trust in the indications of their senses as infallible guides, 

 we would recommend the particular study of this class of 

 deceptions. They will here find their judgments deluded, 

 where every thing is favourable to the discovery of the 

 truth ; and even when they are aware of the source of the 

 deception, they will find themselves again brought under 

 its dominion, and again released from it, by the opera- 

 tion of the most trivial circumstances which they are not 

 able to discover, and the influence of which, if they do 

 discover them, they are not able to appreciate. If all this 

 takes place in matters of simple observation, where the senses 

 of sight and of touch are allowed their undisturbed exercise, 

 how much more liable must they be to error, where their pas- 

 sions, their prejudices, or their feelings, concur in promoting 

 the delusion, or even in any remote degree prepare the mind 

 for its reception. 



The class of deceptions to which we allude, were, so far as 

 we know, first noticed at one of the early meetings of the 

 Royal Society of London, when a compound microscope, on a 

 new construction, was exhibited. When the members were 

 looking through it at a guinea, some of them saw the head up- 

 on the coin depressed, while others considered it to be raised, 

 as it was in reality. 



This deception was studied by Dr Philip Frederick Gmelin 

 of Wurtemburg, who communicated the following observa- 

 tions upon it to the Royal Society of London in 1744. 



" Reing informed by a friend, says he, that if a common seal 

 was applied to the focus of a compound microscope, or opti- 

 cal tube, which has two or three convex or plano-convex 

 lenses, that part which is cut the deepest in it would ap- 

 pear very convex, and so on the contrary ; and that some- 

 times, but very seldom it would appear in the same state as to 

 the naked eye. I was desirous to make the observation 

 myself, and found it constantly to happen as my friend told 

 me. I thought the experiment worthy of being farther pro- 

 secuted ; and, accordingly, on the 16th of last April, the 



