106 INTRODUCTION TO GENERAL SCIENCE 



tensity of illumination does not fall off proportionally to the 

 distance, but to the distance squared. Thus a light at three 

 feet gives but one-ninth the intensity of the same light at a 

 distance of one foot. 



If a standard candle and another source of light give the 

 same illumination to two blocks of paraffin, separated by tin- 

 foil, and placed between the two sources of light, both blocks 

 appear the same shade. The distances of the two lights from 

 the block may be measured, and the intensity of the unknown 

 light may be computed. 



Two shadows of a rod may be obtained side by side, one 

 from a standard candle, and the other from the source of light 

 which is to be measured. The distance of the unknown 

 may be varied until the two shadows have the same density. 

 Then the distances may be measured and the unknown candle 

 power computed. Instruments such as those described are 

 called photometers. 



References* 



1. 1803 : 392-395. Intensity of Light Candle Power. 



a. 1801 : 193-195. Photometry. 



6. 1802 : 208-211. Measurement of Light the Photometer. 



c. 1804 : 327-329. Intensity of Illumination Photometry. 



d. 1805 : 221-224. Intensity of Light Photometry. 



e. 1806 : 144-146. Measurement of Light Photometry. 



/. 1807:263-265. Intensity of Illumination The Photom- 

 eter. 



g. 1808 : 377-379. Intensity of Illumination Photometry. 

 h. 1809 : 279-281. Illumination, Photometry, Photometers. 

 i. 1810 : 333-337. Photometry, Photometers, Candle Power. 



Experiment 38. Candle Power The Photometer. 

 Apparatus: A block of wood 3"X3"X1" with a peg 

 6" X 1" inserted in one side, two blocks 3" X 2" X I", meter 



