PHOTOMETRY AND ELECTRIC LIGHTING. 



315 



0.6 



17-6 



25.6 



30.0 



36.6 



shows the distribution of light about a typical " i6-candle- 

 power " carbon-filament lamp without a shade, and Fig. 176 

 shows the distribution of light about the same lamp when it is 

 placed in an aluminum cone reflector. In these figures the con- 

 ical intensity of the light in each di- , 

 rection in candles is represented to 

 scale by the length of the corre- 

 sponding radius vector of the dotted 

 curve. 



The distribution of light about a 

 lamp, which, like a carbon-filament 

 electric lamp, can be held in any 

 position, may be determined by 

 mounting the lamp in a universal 

 holder at one end of the photometer 

 bar, turning it step by step into 

 various positions, and taking the 

 photometer reading for each position. 



In some cases a lamp is sym- 

 metrical with respect to an axis, 

 so that a complete knowledge of the distribution of light about 

 the lamp may be obtained by determining the intensities of the 

 light in different directions in a single plane which contains the 

 axis of symmetry. 



In many cases a lamp is approximately symmetrical with re- 

 spect to an axis so that the slight variations of the intensity 

 of the light around the axis of approximate symmetry are 

 of no importance. In such a case the lack of symmetry 

 may be averaged out, as it were, by rotating the lamp at 

 a speed of three or four revolutions per second about its axis 

 of approximate symmetry while the photometric readings are 

 being taken. The data for Figs. 175 and 176 were obtained 

 in this way. 



In the case of a lamp which must be held in a fixed position, 

 one or more mirrors are used to reflect the different beams from 



I 



Fig. 176. 



