Instead of a piece of white paper, hold a piece of _black cloth_ two or three inches from the window-pane, and you will have two reflections so weak that the image of the cloth will be almost lost. The first reflection is that of the very small amount of light from the black surface on to the glass, and the second reflection is that of the inconceivably small amount returned by the glass, and by which the faint image of the black cloth is produced. But put the black cloth outside of the window-pane, and then hold an object before them, and you will find that the _two weak reflectors, acting together_, produce an improved image, or reflection. 508. _Why, if a book is held between a candle-light and the wall, does a shadow fall upon the wall?_ Because the rays of light are _intercepted_ by the book. 509. _Why do the rays pass over the edges of the book in a direct line with the flame of the candle?_ Because light always travels in _straight lines_. 510. _Why is there some amount of light even where shadows fall?_ Because, _as all objects reflect light_, some of them throw their light into the field of the shadow. 511. _Why are some substances opaque to light?_ Because the arrangement of their particles will not admit of the _vibrations of the luminous ether_ passing through them. Opaque--impervious to rays of light. 512. _Why do we see our faces reflected in mirrors?_ Because the rays of light from our faces are _reflected_ by the surface of the _quicksilver_ at the back of the glass. [Verse: "The day is thine, the night also is thine: thou hast prepared the light and the sun."--PSALM LXXIV.] 513. _Why does the quicksilver reflect the rays of light?_ Because, being _densely opaque to light_, and presenting also a bright surface, it is a good reflector, and it _throws back the whole of the rays_. 514. _What has the glass to do with the reflection?_ The glass has _nothing to do with the reflection_, except that it affords a field upon which the reflecting surface of the quicksilver is spread; and it keeps the air and dirt from _dulling the quicksilver_. The parts of a mirror from which the quicksilver is rubbed away give no reflection that could assist the reflecting power of the quicksilver. That the surface of the glass does not reflect the image, is shown by the fact, that if you put the point of any object against the glass, the thickness between the point and the place where the reflection of it begins, will _show the exact thickness of the glass_. 515. _Why does a compound mirror (a multiplying mirror) exhibit a large number of images of one object._ Because all objects reflect rays of light in _every direction_, and therefore the different mirrors, being at _various angles_, receive _each a reflection_ of the same object. 516. _Why does a window-pane appear to be a better reflector by candle-light than by day-light?_