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sided, and concave. There is one property, however, 

 mentioned by Aulus Gellius, which has given unnecessary 

 perplexity to commentators. He states that there were 

 specula which, when put in a particular place, gave no 

 images of objects, but, when carried to another place, 

 recovered their property of reflection.* M. Salverte is of 

 opinion that, in quoting Varro, Aulus Gellius was not 

 sufficiently acquainted with the subject, and erred in 

 supposing that the phenomenon depended on the place 

 instead of the position of the mirror ; but this criticism is 

 obviously made with the view of supporting an opinion of 

 his own, that the property in question may be analogous 

 to the phenomenon of polarised light, which at a certain 

 angle refuses to suffer reflection from particular bodies. 

 If this idea has any foundation, the mirror must have been 

 of glass or some other body not metallic, or, to speak 

 more correctly, there must have been two such mirsors, so 

 nicely adjusted not only to one another, but to the light 

 incident upon each, that the effect could not possibly be 

 produced but by a philosopher thoroughly acquainted 

 with the modern discovery of the polarisation of light by 

 reflection. Without seeking for so profound an explana- 

 tion of the phenomenon, we may readily understand how 

 a silver mirror may instantly lose its reflecting power, in 

 a damp atmosphere, in consequence of the precipitation of 

 moisture upon its surface, and may immediately recover it 

 when transported into drier air. 



One of the simplest instruments of optical deception is 

 the plane mirror, and when two are combined for this 

 purpose it has been called the magician's mirror. An 

 observer in front of a plane mirror sees a distinct image 

 of himself ; but if two persons take up a mirror, and if 

 the one person is as much to one side of a line perpen- 



* Ut speculum in loco certo pos'dum nihil imaginet ; aliorsum trans- 

 latum facial imagines. Aul. Gel. Xoct. Attic, lib. xvi. cap. 18. 



