224 GENERAL SCIENCE 



six candles to the pound. A photometer is an apparatus 

 for measuring the candle power of light sources. 



Bunsen Photometer. Place a drop of oil on a sheet 

 of unglazed white paper. When we hold the paper 

 between the eye and the window, the spot appears light. 

 When held so that the eye is between the paper and the 

 window, the spot appears dark. By experimenting we 

 learn that when the paper is viewed from the side of the 

 greater illumination the oiled spot appears dark but when 

 viewed from the other side it appears light. 



Experiment 64. Place a lighted candle A at a distance of two 

 decimeters from a screen S having in its center a square of oiled 



paper (Figure 197). 

 0n the other side of 

 the screen place the 

 light D to be meas- 

 ured, the light in this 

 case being that of an 

 electric bulb. Move 

 FIG. 197. A Simple Arrangement for Measuring J) un til a position is 

 the Intensity of Light. 



oiled paper appears the same from either side. The square of the 

 distance from S to D divided by the square of the distance from 

 A to S will give the candle power of the light D. Suppose the 

 distance from S to D to be eight decimeters. Then the candle 

 power of D is 16, since 64 divided by 4 is 16. The room must 

 be free from light sources excepting those used in the experiment. 



Reflection of Light. An ordinary piece of glass both 

 reflects and transmits light. Many of us have noticed 

 the reflection of the open fire by the window-panes, making 

 it appear as if there were another fire outside. On the 

 outside of the window the fire may be seen by transmitted 

 light. 



Hold a mirror so that the sun's rays falling upon it 

 will appear as a spot of light on the wall or ceiling of the 



