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THE POPULAR SCIENCE MONTHLY 



minated, throw back the light in all directions, and this is known as 

 irregular reflection or diffusion. The effect of regular reflection, 

 which depends upon the polish of surfaces, is not to make those sur- 

 faces visible, but to exhibit images of surrounding objects ; it is by 

 the light irregularly reflected upon their surfaces that objects are seen. 

 In looking into a mirror, the image of the face is seen by regular re- 

 flection ; the surface of the mirror is recognized by irregular reflec- 

 tion. " The mirrors of the ancients were of metal, usually of the 

 compound now known as speculum-metal. Looking-glasses date from 

 the twelfth century. They are plates of glass, coated at the back 

 with an amalgam of quicksilver and tin, which forms the reflecting 

 surface. This arrangement has the great advantage of excluding the 

 air, and thus preventing oxidation. It is attended, however, with the 

 disadvantage that the surface of the glass and the surface of the amal- 

 gam form two mirrors ; and the superposition of the two sets of 

 images produces a confusion which would be intolerable in delicate 

 optical arrangements. The mirrors, or specula as they are called, of 

 reflecting telescopes, are usually made of speculum-metal, which is a 



Refraction by Watek. 



bronze composed of about thirty-two parts of copper to fifteen of tin. 

 Lead, antimony, and arsenic, are sometimes added. Of late years 

 specula of glass coated \ti front with real silver have been extensively 

 used; they are known as silvered specula. A coating of platinum has 

 also been tried, but not with much success." 



It is well known that the effect of plane mirrors or of any polished 

 plane surface is to produce behind them images exactly similar both 



