62 



FIRST YEAR SCIENCE 



ordinary incandescent electric light is sixteen candle 

 power. 



No comprehensible figures will express the intensity of 

 the sun, using the candle power as a measure. The inten- 

 sity of light, like that of heat and electricity, and all forms 

 of energy which spread out uniformly from their point of 

 origin, varies inversely as the square of the distance from 

 the source. This rapid decrease in the brightness of light 



Fig. 36. 







as the distance increases is the reason why so small a 

 change in the distance of a lamp makes so great a differ- 

 ence in the ease with which we can read a book. If we 

 make the distance to the lamp half as great, we increase 

 the amount of light on the book four times (Fig. 36). 



32, Reflection of Heat and Light. Experiment 33. In a 

 darkened room reflect by means of a mirror, a ray of light from a 

 small hole in the curtain, or from some artificial source of light, on to 

 a plane mirror lying flat upon a table. If there is not sufficient dust 

 in the air to make the paths of the rays apparent, strike two black- 

 board erasers together near the mirror. Hold a pencil vertical to the 

 mirror at the point where the rays strike it. Compare with each 

 other the angle formed by each ray with the pencil. Raise the edge 

 of the mirror, and notice the effect on the reflected ray. Place the 

 pencil at right angles to this new position of the mirror, and compare 

 the angles in each case. How do the sizes of the angles on either side 

 of the pencil compare ? 



It has already been stated that the moon shines by re- 

 flected light. It is a matter of common observation that 



