DEVICES FOR SEEING 283 



a flame of uniform intensity. The intensity of the light from 

 an ordinary candle is quite variable. 



Suppose we wish to measure the candle power of an electric 

 light of unknown power. We may stand a nail or similar object 

 upright on the table so its shadow will fall on a white paper or 

 a ground-glass screen. Then place a lighted standard candle 

 on the table so it will throw a shadow beside that made by the 

 electric light. Move the candle nearer to or farther from the 

 nail until the two shadows are equally dark. The comparison 

 is easily made when the shadows are side by side on the paper 

 or screen. Suppose the candle is then i foot from the nail and 

 the electric light is 10 feet away. The relative intensity of the 

 two lights is as the square of these distances. The electric 

 light is, therefore, one of 100 candle power. (An ordinary 

 candle may be used to show the principle of the experiment, 

 but the result will not be exact.) 



Another interesting application of this principle that light 

 travels in straight lines is seen in the pinhole camera. This 

 may be made as follows. Secure a small light-tight wooden 

 or pasteboard box a starch box or chalk box. In the center 

 of one end bore a tiny hole, like a pinhole. Cut out the other 

 end of the box, and over the opening fasten a piece of white 

 tissue paper or, better still, tracing paper or tracing cloth. Set 

 this box on the sill of an open window, pinhole out. Throw 

 a dark cloth or your coat over your head and also over the end 

 of the box covered with the tracing paper. Hold the cloth or 

 coat tightly around the box so that no light gets to your eyes. 

 Look, now, on the tracing paper and you will see an inverted 

 image of the landscape in front of the camera. Every point in 

 that landscape is sending a tiny beam of light in a straight line 

 through the pinhole to the paper to make a part of the image (Fig. 

 132). If a second hole were punched near the first, another image 

 would be formed that would overlap and blur the first. Then if 

 the hole made in the end of the box is large instead of small like 

 a pin prick the overlapping images are all indistinct, and the 



