838 SIR W. DE W. ABNEY : MODIFIED APPARATUS FOR MEASUREMENT OF COLOUR 



complementary colour was found which, when mixed with the former, gave a match 

 to white. The luminosity of each was taken separately when the match was 

 complete. The composition of the part of the spectrum from the yellow to orange 

 was already known, and the complementary colour found was converted into the 

 percentage components of red and green. This enabled an equation in two standard 

 colours and the unknown colour to be formed. When equated to the standard 

 equation, the percentage composition of the last was found. When the colours 

 in one spectrum approach the violet, the movement of the scale in the other 

 spectrum to find the complementary colours is very small, and the magnified scale, 

 formed as described, was of great assistance, since a vernier could be used when 

 required. 



(8.) The Composition of the Violet. 



This remains as given in my previous paper of 1899. Many measures were made 

 in this region of the spectrum, and the percentage of red to blue remains as before. 

 The following are some of the principal measures : 



(9.) Inherent White Light in SSN 37 '5. 



To find the amount of inherent white in SSN (standard scale number) 37 '5. 

 Taking an orange below D at SSN 50'2, it is found that in luminosities 



RS. 37-5. Orange. White. 

 41 + 55 = 57 + 39. 



As there is no white in RS (red sensation), it follows that the 39 white is in 



55 (37'5), or that the 



Orange. RS. GS. 



57 = 41 + 16, 



and that there is |f of GS (green sensation) in 55 (37 - 5); that is, there is 29'5 per 

 cent, of GS in (37 '5). 



Taking another orange near D at SSN 50 '05, it was found that 



RS. (37-5.) Orange. White. 



487 + 45-8 = 63 + 31'5, 

 as before 



RS. GS. 



487 + 14-3 = 63. 



That is to say, there is 31 '2 per cent, of GS in (37'5). Other measures, and they 

 were many, gave 



31 per cent., 3T8 per cent., 30'8 per cent., 29'8 per cent, 32'4 per cent., &c., 



and the mean gave 31 per cent, (very closely) of green sensation in the colour, and 

 this number was adopted. 



