Photometry of Lights of Different Colours. 855 



the field is white and is illuminated by auxiliary carbon 

 lamps. The speed counter, slit mechanism, &c, have pre- 

 viously been described in detail. The light source whose 

 spectrum was measured was, as before, a tungsten lamp ; 

 the details of its use are given below. 



Details of Measurements. 



Energy Distinbution in Spectrum of Light Source. — The 

 spectral luminosity curve which is desired is that of a known 

 energy distribution, from which the luminosity curve of any 

 other distribution may be derived. For convenience it is 

 desirable to give the results for an equal energy spectrum. 

 Were it feasible to measure the intensity of radiation directly 

 at the eye-slit this would be the ideal measurement, since it 

 would at once eliminate the determination of various instru- 

 ment corrections. In the present investigation this was not 

 considered practicable, so measurements were made on a set 

 of four frosted bulb tungsten lamps whose energy distribution 

 was determined as follows : — All four lamps (previously 

 seasoned) were carefully colour-matched on a photometer 

 against a tungsten standard of 1*15 watts per mean horizontal 

 candle-power. Immediately before and immediately after 

 the use of each lamp on the flicker spectrometer it was 

 measured on a spectrophotometer (substitution method) 

 against a tungsten lamp which had previously been measured 

 against a black body of known temperature *. This latter 

 lamp had a visual energy distribution corresponding to a 

 black body at 2295° abs. The frosted lamps were compared 

 with it at four points in the spectrum. The distribution for 

 a black body, 



J14500 



J^ = Ci\~ 5 e at 5 



was plotted as a straight line in the form 



, J K 14500 , 

 Io ^5=~xT- log10 '- 



The spectrophotometer ratios were plotted (in logarithmic 

 units) against this straight line ; through these points 

 another straight line could be drawn if the new energy 

 distribution corresponded to another black body colour, as 

 is pretty well established. This was found to be the case, 

 and from the slope of this line the equivalent black body 



* This standard of black body distribution was obtained and has been 

 maintained for this Laboratory by Dr. E. P. Hyde. 



