ANTERIOR AND POSTERIOR SURFACES. 



lively ; and two other images will be formed at equal distances 

 beyond these latter. In the same way we shall have, by each 



Fig. 3. 



pair of images becoming objects to the respective mirrors, an 

 indefinite series of equidistant images. 



6. The distance between each successive pair of images will be 

 equal to the distance of the object o from either of the images 

 o' or o', and consequently to the distance between the mirrors. 



These effects may be seen in rooms where two looking-glasses 

 are attached to opposite and parallel walls, with a lamp or chan- 

 delier suspended between them. In such cases an indefinite row 

 of lamps will be seen in each glass, each becoming fainter and 

 fainter as the images are more distant. 



7. In the preceding explanation of the effects produced by 

 plane mirrors, it has been assumed that the reflections by which 

 the phenomena are produced, take place from the surface of the 

 mirror. In the case, however, of mirrors made of glass in the 

 usual manner, there are two surfaces, the anterior and the pos- 

 terior, which ought to be, and in good glasses always are, truly 

 parallel. The posterior surface is coated with a metallic compo- 

 sition called an amalgam of tin, which consists .of a combination 

 of tin and mercury, produced by diffusing mercury on the well- 

 cleaned surface of the glass, and then laying upon it a sheet of 

 tin foil. The mercury immediately combines with the tin, 

 forming an amalgam which closely adheres to the glass, and 

 forms a perfectly opaque coating upon it. 



8. When the ray of light proceeding from any object placed 

 before the glass falls upon its anterior surface, it is resolved into 

 three parts to which severally it is necessary to give especial 

 attention inasmuch as the quality and goodness of the looking- 

 glass altogether depends on them. 



The first and principal part enters the surface, and being 

 refracted by the glass passes through it to the posterior surface. 

 What happens to this part we shall presently see. 



The second part is reflected from the anterior surface according 

 to the laws of reflection already explained, and produces an image 

 visible to an observer in front of the glass. 



The third is reflected from the surface of the glass not according 

 to the laws of reflection explained above, but in the same manner 



123 



