COMMON THINGS THE LOOKING-GLASS. 



as that in which light is reflected from the surface of ground- 

 glass. It is this part of the reflected light which renders the 

 anterior surface of the glass visihle. 



9. Since the perfection of the illusion which the looking-glass 

 is intended to produce requires that the spectator, who directs his 

 view to the glass, should see nothing except the optical image of 

 the scene which is before the glass, it is evident that the last 

 mentioned part of the incident light must necessarily have a 

 tendency to destroy the illusion by rendering the surface of the 

 glass itself visible. The less therefore the portion of the light 

 is, which is reflected in this manner, the more perfect will be the 

 glass. When a glass is highly polished, and perfectly free from 

 scratches, the part of the light thus radiated from its sur- 

 face, though not strictly speaking nothing, is nevertheless so 

 exceedingly small as to produce no sensible effect on the eye. 

 So great is the perfection to which the surface of plate-glass has 

 been brought in this respect, that a plate of looking-glass brought 

 down to the surface of the carpet in a room produces so perfect an 

 illusion, that a person with good sight would take it for an open 

 door and walk through it. 



10. Large mirrors thus set in rooms flush with the carpet, and 

 surrounded with framing fashioned like that of open folding- 

 doors, have the apparent effect of converting a single room into a 

 vast suite of rooms, and when the mirrors are of good quality the 

 illusion is so complete, that persons are only prevented from 

 attempting to walk from room to room by meeting their own 

 image in the door- way, which generally excites a sensation of 

 indescribable surprise.* 



"When, however, glasses have the polish of their surface more or 

 less deteriorated by long use, and above all by constant and 

 improper cleaning, the part of the light radiated in this manner is 

 so much increased that their surfaces become visible, especially 

 when they are viewed obliquely. 



11. "Where valuable glasses require frequent cleaning great 

 care should be taken as to the manner in which the operation is 

 performed, since otherwise they will soon lose their polish and be 

 very much deteriorated in beauty and diminished in value. The 

 dust which collects upon them should be first removed by means 

 of a duster of feathers,f and they should then be cleaned either 

 with wash-leather or old cambric. Nothing can be more 



* I have made this optical experiment in one of the rooms of my own 

 house, and have often observed the result above described. 



*f Called in France a plumeau. I am informed that this article of 

 household convenience does not exist in England ; nevertheless, it is of 

 great and constant utility elsewhere as an instrument of domestic neatness. 

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