54 INTENSITY OF LIGHT. 



CHAPTER VL 



INTENSITY OF LIGHT. 



THE art of measuring the intensity of light 5s called 

 photometry. The word comes from the Greek word pho- 

 tos, which means of light, and the word metros, measure- 

 ment. On the same principle, the word photometer would 

 mean a light-measurer, just as thermometer is a heat-meas- 

 urer, and barometer a weight-measurer, and dynamometer 

 a strength -measurer, from Greek words meaning heat, 

 weight, and strength. 



Some very curious devices have been contrived for meas- 

 uring the comparative intensity of different lights. In some 

 of thes-e devices the observation is made by examining the 

 shadows cast by the two sources of light to be compared. 

 How this is done is shown by the engraving. There is a 

 stand with an upright rod (m) fitted to it, and beyond the 

 rod a screen, made usually of a plate of ground glass, to 

 receive the shadows. Any white surface would answer 

 well enough for such a screen, but ground glass is found 

 to possess some peculiar advantages for this purpose. 



The two lights to be compared are placed at a distance 

 from the upright rod on the side opposite to the screen, so 

 as to cast the shadows a and d upon it. 



In the engraving the sources of light are a lamp (L) and 

 a candle (B). The shadows seem to be of nearly the same 

 intensity. If, on careful examination, they are found to be 

 as nearly as possible alike, and if the lamp, as would seein 

 to be the case, is nearly twice as far from the screen as the 

 candle, then it would show that the light from the lamp 



