COMMON THINGS THE LOOKING-GLASS. 



reddish tints be absorbed in excess, the objects reflected will have 

 a tendency to greenish or bluish tints, 



19. A good looking-glass should have its two surfaces truly 

 parallel and truly plane. If they are not truly parallel, the sepa- 

 ration of the images produced by the anterior and posterior sur- 

 faces will be augmented, and confusion will be produced. If the 

 surfaces be not truly plane, the relative position of the images, 

 and of different parts of the same image, will not correspond with 

 that of the objects and parts of the same object, and consequently 

 distortion will ensue. 



20. A good mirror plate must also be perfectly homogeneous, 

 since otherwise the rays would be differently refracted in passing 

 through it, as well from front to back as from back to front, and 

 consequently distortion would in that case also be produced. 



These defects may generally be observed to prevail more or less 

 in the more common and low-priced sorts of looking-glasses, which 

 are often so striated as to reflect images utterly distorted. 



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