IMAGE IN THE GLASS. 



obedient, and most perfect ministrations, are in the least degree 

 aware of the admirable optical experiment which they perform, 

 nor of the principle upon which so faithful a reproduction of their 

 person and lineaments depends, it will neither be unprofitable nor 

 uninteresting, to place before our readers a brief exposition of the 

 THEORY OF THE LOOKING-GLASS. 



2. It has been already shown in our Tract upon "Optical 

 Images," that when an object is placed before a plane reflector, 

 the rays of light which diverge from each point of its surface, 

 after falling upon the reflector, will be thrown back, or reflected 

 as if they had proceeded from a point as far behind the reflector 

 as the point from which they did actually proceed, is before it. 

 It follows from this, that an observer in front of the reflector, 

 receiving in this manner the reflected light, as if it came from a 

 similar object behind the reflector, will have a perception of such 

 an object. Thus, when he stands at a certain distance before the 

 reflector, as in fig. 1, he sees his own image standing face to face 

 with him, just as far behind the reflector as he is before it. The 

 head of the image will correspond in position with his head, and 

 the feet with his feet, that is to say, the image will be erect and 

 not turned upside down, as is the case with some other optical 

 images, as may be seen by reference to our Tract on that subject. 

 But an inversion will be produced when the image is considered 

 laterally ; this will be understood when it is considered that the 

 observer and his image are looking in opposite directions since 

 they stand face to face. If the observer, for example, look to the 

 north, his image looks to the south, and in that case the right 

 hand of the observer would be to the east, and his left to the 

 west, while the right hand of his image would on the contrary be 

 to the west, and the left to the east. Thus, the reflection of the 

 right hand of the observer would be the left hand of his image, 

 and the reflection of his left hand would be the right hand of the 

 image. 



This effect is rendered strikingly manifest by holding before a 

 reflector a printed book. On the image of the book all the 

 letters will be reversed. 



3. If an object be parallel to the surface of a looking-glass, its 

 image will also be parallel to it, for since in that case all parts of 

 the object are equally distant from the reflecting surface, all 

 parts of the image must be also equally distant from it. 



4. It follows also, from what has been explained, that if an 

 object be not parallel to a reflector, but forms an angle with it, 

 the image will form a like angle with it, and will form double 

 that angle with the direction of the object* 



Let A B, fig. 2, be a plane reflector, before which an object c :> 



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