ON THE THEORY OF COMPOUND COLOURS. 439 



and the value of D, as deduced from these observers, would have been 



23 (104) -17 (88) -I'l (68) =D (5), 



in which the defective sensation is much nearer to the red of the spectrum. It 

 is probably a colour to which the extreme red of the spectrum tends, and 

 which differs from the extreme red only in not containing that small proportion 

 of " yellow " light which renders it visible to the colour-blind. 



From other observations by Mr Simpson the following results have been 

 deduced : 



TABLE a. 



(88.) (68.) 



(99-2 + )= 33-7 1-9 



31-3(96)= 33-7 2-1 



28 (92)= 33-7 1-4 



33-7(88)= 33-7 



54-7(84)= 33-7 6-1 



71 (82)= 33-7 15-1 



99 (80)= 33-7 33-1 



70 (78)= 15-7 33-1 



56 (76)= 5-7 33-1 



36 (72)= - 0-3 33-1 



33-1(68)= 33-1 



40 (64)= 0-2 33-1 



55-5(60)= 1-7 33-1 



(57 - ) = - 0-3 33-1 



'' (88.) (68.) 



100(96)= 108 7 



100(92)= 120 5 



100(88)= 100 



100(84)= 61 11 



100(82)= 47 21 



100(80)= 34 33 



100(78)= 22 47 



100(76)= 10 59 



100(72)= 1 92 



100(68)= 100 



100(64)= 83 



100(60)= 3 60 



In the Table on the left side (99'2 + ) means the whole of the spectrum beyond 

 (99'2) on the scale, and (57 ) means the whole beyond (57) on the scale. The 

 position of the fixed lines with reference to the scale was as follows : 



A, 116; a, 112; B, 110; C, 106; D, 98'3 ; E, 88; F, 79; G, 61; H, 44. 



The values of the standard colours in different parts of the spectrum are given 

 on the right side of the above Table, and are represented by the curves of 

 fig. 9, Plate VII. p. 444, where the left-hand curve represents the intensity 

 of the " yellow " element, and the right-hand curve that of the " blue " element 

 of colour as it appears to the colour-blind. 



The appearance of the spectrum to the colour-blind is as follows : 

 From A to E the colour is pure "yellow" very faint up to D, and 

 reaching a maximum between D and E. From E to one-third beyond F towards 



