68 Measurement of the Luminosity and Intensity of Light. 



[The scale-numbers M'hicli apply to the Fraimhofer-lines are 

 as follows :— A = 42-56, B = 43-34, C = 44-26, D = 46-54, 

 E =49-62, F = 52-08, G-=57-72.] 



The areas of the curves are those shown in the table. 

 Taking- the area of the luminosity curve of white as 100, 

 the following are the luminosities of the colours derived from 

 the above : — • 



White. Yellow. Orange. Red. Blue. Green. Neutral Tint. 

 100 84 60 41 20 34 41 



The readings by the new method were : — 

 White. Yellow. Orange. Red. Blue. Green. Neutral Tint. 



77 65 45 31 15-5 27 31 



Taking white as 100, as before, the readings reduce to : — 

 White. Yellow. Orange. Red. Blue. Green. Neutral Tint. 



100 84 58 40 20 35 40 



The exactitude of the latest method is again exemplified, 

 but it is not pretended that it is quite so accurate as the 

 older method. 



The method described is particularly adapted to a coloured . 

 surface of small dimensions. Where, however, a circle of 

 some 3 or 4 inches in diameter can be utilized, the following 

 modification of Rood's method may be adopted. Behind the 

 sectors and on the spindle of the rotating disks, a disk of 

 white card can be placed. The sectors being blackened on the 

 other side, when revolved we can produce any shade of grey, 

 from half black and half white to all black. On the other 

 side of the sectors can be placed a smaller disk of coloured 

 card, to be measured, and this will rotate with the sectors. 

 Should the colour be a fairly dull one, the match of luminosity 

 can be made by altering the shade of grey by rapid oscillation 

 in aperture of the sectors, and the luminosity of the colour 

 in terms of white determined when the amount of white 

 light reflected from the black surface has been measured. 

 Should the colour be more luminous than the grey given by 

 half black and half white, the coloured disk can be toned 

 down by means of a black semi-disk^ adding a known quantity 

 of the black surface by Maxwell's plan. The luminosities of 

 the surfaces are then measured, and the true luminosity of the 

 coloured surface in terms of the white surface, calculated after 

 allowing for the added black. 



This same plan may be adopted for measuring the intensity 

 of the light of the various parts of the spectrum reflected from 

 a coloured surface, in comparison with that reflected from a 

 white surface. Colour patches of monochromatic light are 



