DOUBLE REFLECTION. 



when this line is perpendicular to the glass, the image s is 

 directly between the eye and s', so that the one is projected on 

 the other, and they are seen as a single image. Hence it arises 

 that when a person looks at himself in a glass, he never in any 

 case can see the double image, since in that case the line of vision 

 must be always at right angles to the glass. But when he views 

 an image of another object from a point such as E, where the line 

 of vision is oblique to the surface of the glass, the images are 

 separated more or less, according to the obliquity of the line of 

 vision. The nearer the eye is to thei*surface of the glass, the 

 more nearly at right angles to's' s will be the line of vision E' o' s', 

 and the more widely will the images appear separated. 



16. The image which is actually seen in the looking-glass is 

 then chiefly that which is reflected by the posterior surface of the 

 glass, and not as many are apt to imagine, that reflected by the 

 anterior surface. If any further evidence of this be required, it 

 will be readily found in the fact that an unsilvered plate of glass- 

 shows by reflection scarcely any perceptible image, although the 

 reflection from its anterior surface is exactly the same as if it 

 were silvered at the posterior surface. The superior brilliancy of 

 the image reflected by the posterior silvered surface is so decided 

 as to overpower the more feeble image produced by the anterior 

 surface, except in the extreme cases of very great obliquity of the 

 visual rays. 



17. All translucent media have the effect of absorbing more or 

 less of the light which is transmitted through them, and to such 

 an extent does this absorption take place, that such media will 

 actually absorb all the light which enters them, if their thickness 

 exceeds a certain limit. It happens also that such a transparent 

 body has a greater tendency to absorb lights of particular tints of 

 colour than others. Thus, while some will absorb a greater pro- 

 portion of the reddish, others will absorb a greater proportion of 

 the bluish tints. 



If the glass of which a mirror is formed have an equal ten- 

 dency to absorb light of all colours, it will reflect all objects 

 placed before it in their natural colours, but rendered somewhat 

 more faint than the objects themselves. The less light is thus 

 absorbed, the more nearly will the reflection correspond with the 

 object, and the more perfect will be the mirror. 



18. If, however, as happens more commonly, the glass have a 

 tendency to absorb particular colours more than others, the object 

 reflected in the glass will appear in false tints, more or less pro- 

 nounced, according to the degree of the absorption, and the colours 

 of the light absorbed. If the bluish tints be absorbed in excess, 

 the objects reflected will be more of a reddish tint ; and if the 



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