6Q Oapt. Abney on the Measurement of the Luminosity and 



chamber. The mirror arrangement was useful, as any varia- 

 tion in the direct light also showed itself in the reflected light. 

 Instead of gas-light, reflected sky -light, the electric light, or 

 sunlight can be employed by very obvious artifices ; for it 

 must be remembered that the comparison-light may be of any 

 kind, and distinct, if necessary, from the light illuminating 

 the coloured surface. 



Greneral Festing and myself had measured with great 

 care the luminosities of emerald green, vermilion, and ultra- 

 marine, which we had combined in a rotation apparatus 

 to produce a grey. The total luminosity of these three colours I 

 measured by the apparatus described, using as the source of 

 light to illuminate the colour, the patch of white light 

 formed by a recombination of the spectrum, the comparison 

 light being an ordinary gas-light. 



The following table gives the values obtained by the spec- 

 trum method and also by the method I now introdrce. It will 

 be seen that the coincidence between the two values is very 

 close : — 



Colour. 



Area of Lumi- 

 nosity Curves 

 (from Paper 

 in the Phil. 

 Trans.). 



Luminosity Readings by 

 New Method. 



Value 

 from 

 areas 

 when 

 white 

 = 100. 



Value 

 from 

 New Plan 

 when 

 white 

 = 100. 



Readings. 



Mean. 



White 



634 

 221 

 216 



49 



73 77,75,76 

 32,28,29,31,31 

 33, 30, 20, 29 



6, 7, 7, 7-5 



75 



30-2 



30-2 



6-9 



100 

 4M 

 40'0 



92 



100 

 40-3 

 40-3 



9-2 



Emerald Green . 



Vermilion 



French Ultra- 

 marine 



For another purpose I had measured the luminosity curves 

 of six different colours by the spectrum method, and below is 

 a table of the intensities of the light of different wave-lengths 

 reflected from each and from a white surface. The areas 

 indicate the total luminosities of the several colours. 



