ON THE THEORY OF COMPOUND COLOURS. 425 



TABLE II. 



Scale. (N+n). Wave-length. Colour. 



20 36-4 2450 Red. 



24 38-3 2328 Scarlet 



28 39-8 2240 Orange. 



32 41-4 2154 Yellow. 



36 42-9 2078 Yellow-Green. 



40 44-3 2013 Green. 



44 45-7 1951 Green. 



48 47 O 1879 Bluish green. 



52 48-3 1846 Blue-green. 



66 49-6 1797 Greenish blue. 



60 50-8 1755 Blua ' 



64 51-8 1721 Blue. 



68 52-8 1688 Blue. 



72 53-7 1660 Indigo. 



76 54-7 1630 Indigo. 



80 55-6 1604 Indigo. 



Having thus selected sixteen distinct points of the spectrum on which to 

 operate, and determined their wave-lengths and apparent colours, I proceeded 

 to ascertain the mathematical relations between these colours in order to lay 

 them down on Newton's diagram. For this purpose I selected three of these 

 as points of reference, namely, those at 24, 44, and 68 of the scale. I chose 

 these points because they are well separated from each other on the scale, and 

 because the colour of the spectrum at these points does not appear to the eye 

 to vary very rapidly, either in hue or brightness, in passing from one point to 

 another. Hence a small error of position will not make so serious an alteration 

 of colour at these points, as if we had taken them at places of rapid variation ; 

 and we may regard the amount of the illumination produced by the light 

 entering through the slits in these positions as sensibly proportional to the 

 breadth of the slits. 



(24) corresponds to a bright scarlet about one-third of the distance from 

 C to D; (44) is a green very near the line E ; and (68) is a blue about one- 

 third of the distance from F to G. 



VOL. i. 54 



