41 S SHADOW. 



be fixed by means of a very thin layer of Canada balsam spread 

 upon the frame, proceeding otherwise as in the case of the 

 writing-paper; but the dry tissue-paper should be carefully 

 tightened upon the frame with the fingers, so as to remove creases 

 as much as possible. The Canada balsam must be allowed to 

 dry a little upon the frame, if it is very thin ; otherwise the 

 paper will not adhere to the frame when the latter is raised. 

 The fingers may be cleaned from the Canada balsam with oil of 

 turpentine. 



The screen may be clamped in the retort-stand whenever required ; 

 or, more conveniently, it may have a handle attached to the frame, 

 10 or 20 cm long and 1'5 or 2 cm thick. The handle is either 

 clamped in the retort-stand, or a little paper is wrapped round it 

 and it is stuck into a candlestick like a candle. 



The fact that the eye placed within the umbra sees nothing of the 

 source of light, while an eye .within the penumbra sees a portion of 

 the luminous body, may be demonstrated by making holes in the 

 screen with a stout pin in the proper places, and applying the eye to 

 the holes, viewing the discs and flame from behind the screen. 



When the screen is close to the opaque body the 

 umbra is nearly of the same size as the latter ; the 

 penumbra forms in this case only a very narrow border 

 round the umbra, and the shadow represents faithfully 

 the outlines of the figure of the opaque body. The 

 smaller the source of light, the narrower becomes the 

 penumbral border, and consequently the more sharply 

 defined is the figure formed by the shadow. 



The earth and the moon are both much smaller than 

 the sun, from which both receive light. They are 

 therefore both accompanied by an umbra, which ex- 

 tends into space and ends at a definite distance from 

 either body, The umbra of the moon extends to a 

 distance of about 240,000 miles, which is nearly equal 

 to the distance of the moon from the earth. If the 

 moon, during its revolution round the earth, comes 

 into a position which is in the straight line between the 



