REFLECTION OP LIGHT. 



2G5 



different looking-glasses will present a more or less agreeable representation 

 of the person who uses them, according to the colors which they may happen 

 to absorb. Thus, if a mirror has a tendency to absorb the red tints, it 

 will give a pallid tint to the complexion ; whereas, if it absorb the blue tints, 

 it will throw a blush over the appearance, and may be called a flattering 

 glass. 



Glass is the most convenient material for mirrors intended for domestic use, 

 because it is the cheapest and most durable ; but it is far from being the best. 

 Its defects will become apparent by considering the mode in which its effects 

 are produced. A coating of metallic foil is attached to the hinder surface of 

 the glass, and by the mode of its adhesion a smooth metallic surface is thus 

 formed under or behind the glass. It is this surface, and not the front of the 

 glass, which is the real mirror : it is by it that the images of objects in front 

 of the looking-glass are produced. The light has to pass through the 

 thickness of the glass to reach this surface, and after being reflected by it, 

 has again to pass through its thickness in order to reach the eye and pro- 

 duce a perception of the image. There are here three successive stages in 

 which light is lost. A part only of the light which strikes upon the front 

 surface of the glass penetrates it, and a part of what does penetrate it is lost 

 upon the hinder surface ; and again, after reflection, in issuing through the 

 front surface, another portion is lost. 



But the loss of light is not the only defect : in passing through the glass, 

 partial absorption of color takes place ; and hence, as has been already stated, 

 the tints of the image will beVdifferent from those of the object. 



A portion of the light which falls on the front surface of the glass is regu- 

 larly reflected, and produces a faint image of the object, which, by careful 

 observation, may be easily distinguished a little in front of the stronger image 

 produced by the silvered surface. The distance of this faint image in front of 

 the other will be equal to the thickness of the glass. 



It is evident, from what has been just observed, that the thinner the glass is, 

 the better will be the mirror. 



The defects which have been just explained have rendered glass reflectors 

 inapplicable to telescopes or any of the class of superior optical instruments 

 used for scientific purposes. In these instruments metallic reflectors alone 

 are used. An alloy of metals is selected for this purpose as white as possible 

 in color, and susceptible of a high polish. A very accurate figure is imparted 

 to it and a very perfect polish by various processes known in the arts. Al- 

 though with such reflectors incomparably less light is lost than in common 

 looking-glasses, still a much greater loss of light takes place than in trans- 

 mission through transparent media ; hence the received maxim in optics, that 

 more light is lost in reflection than in refraction. Liquid surfaces afford in 

 general, when at rest, good plane reflectors. If the liquid be opaque, the 

 reflection is very perfect. This will be rendered apparent by pouring some 

 clear quicksilver on a plate ; to exhibit this effect, the quicksilver should be 

 strained through a piece of chamois leather : it would otherwise have a 

 film upon it composed of foreign matter, which would destroy its reflecting 

 power. 



The objects on the banks of a calm river or a tranquil lake will be seen 

 reflected in its surface ; but it is worthy of notice that the observer can only 

 see this reflection when he looks very obliquely at the surface of the water : 

 the reason of which is, that the rays which strike nearly at right angl<-s to 

 the water penetrate it in virtue of its transparency. It is only those which 

 glance obliquely on it that are reflected ; just as a stone which, thrown per- 

 pendicularly on the water, would immediately sink, will, if projected at a 



