120 Sir "NV. <! W. Abney. On the Estimation of the 



Fn;. L'. 





IT" are jellow discs. 



B* is a black disc. 



W ., white disc. 



is the nut of the spindle. 



From the angles which the sectors of the colours subtend and of the 

 black and white employed, the luminosity of the blue can be calcu- 

 lated. The luminosity of the blue being ascertained, a red disc may 

 be interlaced with the green and the blue disc, and that of the red 

 calculated. As a check a black and yellow disc may be interlaced and 

 compared with the colour given with the red and green discs inter- 

 laced, one of the pairs of course being of greater diameter than the 

 other. 



To ascertain what degree of accuracy could be attained the fol- 

 lowing experiment is given in detail. The light used was the arc light, 

 and the measurements as described above made. 



It was found that the black reflected 3*33 per cent, of white 

 light, and that when the luminosity of the yellow was matched the 

 interlaced black and white discs occupied 82 and 278 respectively of 

 the compound disc. This gave the yellow a luminosity of 78, white 

 being 100. In a similar way the luminosity of emerald green was 

 found to be 43. These two discs were interlaced with a dark blue disc 

 and a grey formed which matched a grey formed by black and white. 

 The following equation was obtained : 



Yellow. Green. Blue. White. Black. White. 



118 + 71 + 171 = 122 + 238 = 130 



