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THE POPULAR SCIENCE MONTHLY. 



the flame ; but, if the eye, the candle, or either screen, be slightly dis- 

 placed to the right or left, the ray is interrupted, and the flame be- 

 comes invisible. 



This propagation of light in straight lines, though the first condi- 

 tion of the production of images, is not the only condition ; for, in that 

 case, the images of illuminated objects would be repeated everywhere, 

 and, when the blinds were opened, a picture of the landscape might be 

 thrown, through the window, upon the opposite wall of a room. For 

 the formation of an image, the rays of an object must be collected and 

 passed through an aperture. This is shown by a simple experiment 

 illustrated in Fig. 2. A card is pierced with a large pin-hole and 

 held between a candle and screen, when an image of the candle will 

 be formed upon the screen in an inverted position. That the image 

 must be upside-down is evident, if the rays take a straight course. A 

 line from the top of the candle-flame through the puncture is pro- 

 longed to the bottom of the image, and another, from the bottom of 

 the flame, crosses the first at the aperture, and strikes the top of the 



Fig. 4. 



Image of Sun upon the Flook. 



image. A line from the centre of the candle passes straight through 

 and strikes the centre of the image, while lines from the two sides of 

 the flame cross again, and are prolonged to the opposite sides of the 

 image. Thus, as each point upon the screen receives only the light 

 from a corresponding point of the flame, the image repeats the object 

 in outline, color, and brightness, though in a reversed position. 



This principle may be applied on a larger scale. Let a room be 

 made quite dark, and a white screen be placed opposite a small hole 



