SHADOW. 417 



the distance of the opaque body from the source of 

 light increases, the penumbra becomes wider and wider, 

 and, properly speaking, reaches to an infinite distance ; 

 but as the distance increases, it becomes fainter and 

 fainter and finally almost imperceptible. 



An eye situate I within the penumbra sees a portion 

 of the source of light, and the more the nearer the eye 

 is to the outer margin of the penumbra. Again, if 

 the eye is in a straight line behind the opaque body, 

 but the latter is smaller than the source of light, then 

 it will hide the central portion of the luminous body, 

 while the edge remains visible. 



Any flame maybe used for these experiments, but the flame of a 

 common parafline lamp is particularly suitable, for it burns brightly, 

 and is rather wide without being too high. Circular pieces of card- 

 board, fixed one over the other upon a knitting-needle, are used for 

 producing the shadows. Of the three discs the upper one should be 

 twice as large as the flame, the middle one of the same size as the 

 flame, and the lower one should be just half as large. The 

 needle is clamped in the retort-stand, and the shadows may be 

 thrown upon the wall, the lamp and stand being placed upon a 

 table close tb the wall. A proper moveable screen is, however, 

 much better, and is useful for other optical experiments. It is best 

 to have a frame made by a joiner for the purpose, and to stretch 

 paper over it; the frame should be from 4 to 6 decimetres high 

 and broad, made of wooden laths, 2 cm wide and 6 or 8 mm thick. 

 Common writing-paper may be used ; but if it is intended to 

 show the experiments to a large audience, tissue-paper is better, 

 for it is translucent, and the shadows may be seen on either side 

 of the screen. The writing-paper must be laid upon the table 

 and damped, before stretching it upon the frame, with a clean 

 moist sponge or cloth ; the frame is then covered on one side 

 with a layer of glue, pressed upon the moist paper, raised with 

 the latter and turned over ; the paper should then be w T ell pressed 

 everywhere upon the frame. When dry the paper will be tight 

 and smooth. Tissue-paper cannot be moistened without tearing 

 it ; it hardly bears the moisture of the glue or gum, and it should 



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