PRECAUTIONS IN CLEANING. 



injurious than whiting applied in the customary way for this 

 purpose. The leather, however, may be impregnated with 

 putty or crocus powder. When the value of large-sized looking- 

 glasses, and their great durability when properly treated, are con- 

 sidered, such precautions for their due preservation will not be 

 considered superfluously extreme. 



12. Let us now return to that part of the light which penetrates 

 the anterior surface, and passing through the glass encounters the 

 posterior or silvered surface. A certain small proportion of this is 

 absorbed by the glass in passing through it, but the chief part 

 arrives at the silvered surface by which it is reflected according 

 to the laws already explained, and returning through the glass, 

 passes through the anterior surface, and issuing from it produces 

 all the phenomena which have been explained in the preceding 

 paragraph. 



13. It follows therefore, that since both surfaces of the glass, 

 the anterior and the posterior, reflect the rays proceeding from 

 any object placed before it, independently of each other, two 

 optical pictures of each object will be produced one in front of the 

 other ; but since the number of rays reflected by the anterior 

 surface is incomparably smaller than the number reflected from 

 the posterior surface, the picture produced by the latter will be 

 proportionately brighter and more visible than the feebler picture 

 produced by the latter. Nevertheless in certain positions of the 

 observer he latter picture will be perceived. 



To render this more easily intelligible let 21 M, fig. 4, be the 

 anterior and M' Ji' the posterior and silvered surface of the glass. 

 Let s be any point of an object placed before it, and let E E' be 

 the eye of an observer viewing this object in the glass. Let us 

 then see how his vision will be affected. 



A ray of light, s o, proceeding from the object, strikes the glass 

 at o. A very small portion of it is reflected to the eye of the 

 observer in the direction o E, so that o E and o s shall make equal 

 angles with the surface M si of the glass. This small portion of 

 light thus reflected in the direction of the line OE causes the 

 observer to see an image, or optical picture of the object, at 5, in 

 the direction of o E, the image s being as far behind the surface 

 M M as the object s is before it. But since the portion of light 

 proceeding from o to E is so very small, the image s will be pro- 

 portionally feeble. 



The chief part of the beam of light entering the glass is 

 refracted in the direction o o', making a very obtuse angle with 

 its original direction s o, and encountering the silvered surface at 

 o', it is reflected back through the glass in the direction o' o, so 

 that the rays o o' and o' o are equally inclined to the surface M' M'. 



125 



