SOLAR IMAGES ON THE PAVEMENT 109 



shape of the luminous patches. But it is not so ; the 

 circular shape greatly preponderates. The explanation is 

 far from difficult. Has the reader ever made for himself a 

 camera obscura ? If not, it is worth while to do it for 

 once. You only want a box of any convenient dimensions, 

 say a foot every way. In the middle of one end pierce a 

 very small hole as accurately circular as possible. It is 

 best to cut a big hole in the wood, cover this with a card, 

 and pierce a small hole in the card with a needle. The 

 opposite end of the box may be taken out and replaced 

 by tracing paper, or it may be papered inside with white 

 paper, so as to form a reflection screen, and a peep-hole 

 cut in one of the adjacent sides. Take the box into a 

 dark room, light a candle, and set it opposite the hole 

 about as far from it on one side as the papered side of 

 the box on the other, that is one foot, if the dimensions 

 given above have been kept. Looking through the tracing 

 paper, or through the peep-hole, you will now see a dis- 

 tinct inverted image of the candle-flame. Why inverted ? 

 A simple diagram, which you can make in a few minutes, 

 will tell you. Draw the box in section, and the candle 

 in front of it. Suppose that the tip of the flame comes 

 2 in. above the level of the hole. Rays of light will pass 

 off in every direction, but only those which enter the hole 

 will take part in the formation of the image. These will 

 all descend, because the hole is lower than the luminous 

 point. Let us next suppose that the bottom of the candle- 

 flame is 2 in. below the level of the hole. The rays 

 reflected from this part of the candle will have to ascend 

 in order to enter the box, and in so doing they will cross 

 the rays emitted by the luminous tip just where both 

 enter the hole. Other light-rays will behave in the same 

 way, descending or ascending according as they issue 

 from points above or below the level of the hole. Your 

 diagram will make it clear that the image formed by the 

 pin-hole must be inverted. Remark that the form of the 

 image is not the same as that of the hole, which is circular, 



