REFLECTION OF LIGHT. 



If an object be placed before a concave mirror at a distance from it greater 

 than that of the geometric centre of its curvature, an image wiA be formed of 

 this object in front of the mirror at a distance from its surface greater than half 

 its radius. 



This image will be inverted, and will be less than the object ; as the object 

 approaches the centre of the curvature of the mirror, the image will also ap- 

 proach that point, and thus the object and image will approach each other ; the 

 image will at the same time be increased in magnitude. If the object be 

 placed within the centre of curvature of the mirror, but farther from its surface 

 than half its radius, a magnified image will be formed at a distance more or 

 less considerable in front of the mirror. Thus, let us suppose that a mirror 

 formed with a curvature having a radius of four feet, has an object in front of it 

 at a distance of three feet from its surface an image of that object will be 

 formed at six feet in front of the mirror, and this image will be double the 

 height or length of the object. 



In this mariner, a mirror placed out of sight of a person may be made to 

 throw the image of an object close to him ; thus a dagger may be presented 

 to one's bosom, which, however, is literally an air-drawn dagger. 



The only form of reflecting surface which presents an object in its natural 

 position and proportions is the plane mimfr commonly used for domestic pur- 

 poses ; and even this, as already explained, reverses the object laterally ma- 

 king right left, and left right. For the purposes, however, to which it is usually 

 applied, this derangement does not impair its utility. 



The perfection with which a mirror presents the image of an object placed 

 before it depends upon its form and material. It is, above all things, essential 

 that its surface should be perfectly plain and even ; any deficiency in this qual 

 ity will produce a corresponding distortion of the image. Cheap looking- 

 glasses are often striated and streaked with inequalities and ridges, which render 

 them nearly useless. Whatever be the substance used to form a mirror, apart 

 only of the light which falls upon it will be instrumental in forming the image. 

 The entire quantity of light which falls on the mirror may be accounted for as 

 follows : 



1 . A part will be regularly reflected according to the laws above explained 

 and it is by this part the image will be formed. 



2. Another part will be irregularly reflected that is to say, it will be scat- 

 tered in every direction around from every part of the surface. It is this por- 

 tion of the light which renders the surface of the mirror visible. 



3. A part will be absorbed upon the reflecting surface and lost. 



The more highly polished and even the reflecting surface is, the less will 

 be the part irregularly reflected, and the brighter will be the image. The part 

 of the light absorbed or stopped will depend on the physical quality of the 

 matter of which the reflector is formed. 



Since art cannot produce a perfect reflecting surface, there will always 

 be a portion of the incident light irregularly reflected and absorbed. It follows, 

 therefore, that light is always lost in reflection ; and in the case of plane mir- 

 rors, where the magnitude of the image is equal to that of the object, the bright- 

 ness of the image must always be less than that of the object. 



There is no substance which reflects with equal facility all tints of color. 

 It generally happens that lights of one tint are more absorbed than the lights 

 of another. Mirrors, therefore, will produce a change more or less according 

 to their degree of imperfection in the tints which characterize the object before 

 them ; in other words, the color or tints of the image will not correspond ex- i 

 actly with those of the object. 



It is therefore a fact true in science, although sometimes ridiculed, that < 



