106 Dr. W. Pole on some unpublished 



the line YU must be divided at in the proportion of 61J to 

 38J, and a line must be drawn from W through and pro- 

 longed downwards. The distance WO being taken as 64 

 parts, the prolongation must be made 36 parts, which will 

 give the point Bk. Then the lines from this point to Y and 

 U will complete the figure. It will now be easy to see how 

 all the varieties of our colour-impressions, above described, 

 may be represented on the diagram. 



The simple impressions 1, 2, 3, 4 are on the angular points. 

 The line YW contains the whole series of class 5 ; the line UW 

 the whole series of class 8 ; the line Y Bk contains the whole 

 series of class 6 ; the line U Bk, that of class 9 ; the line 

 TV Bk contains the whole series of greys in class 11. The 

 position of a point representing any given mixture along 

 these lines is fixed by the usual Newton's rules ; thus a 

 mixture of half yellow and half white would lie halfway 

 between Y and TV. A mixture of one part yellow and 3 

 parts black would lie at a distance from Bk of one fourth the 

 length of the line. 



The impressions in classes 7 and 10 are represented by 

 points lying in the interior of the figure ; No. 7 (yellow) 

 lying all on the left-hand side of the grey line ; No. 10 

 (blue) all on the right-hand side. The mode of finding the 

 position of any point will be best shown by an example : 

 take the emerald green as defined by my equation IX. First 

 divide the grey line into (58 + 19 = ) 77 parts, of which set 

 off 19 at the lower end, giving a point a. From this draw 

 a line aY, divide it into 100 parts and set off 23 of these 

 from a. This will give the point G, representing my emerald 

 green. The various quantities relating to this impression may 

 be determined geometrically as follows : — Draw a line from TV 

 through the colour-point G, cutting the line Y Bk in s ; then 



s~Gc 

 the degree of saturation = 1 — ==■ . Similarly draw a line from 



sTy 



Bk through G, cutting WY in /, then the degree of luminosity 



= 1 — =■ , and the chromic strength = ;=. 

 IB ' ^ aY 



If a line be drawn from vermilion, or from any red colour, 

 to U, the point where it crosses the line TV Bk will represent 

 a colour which, though a powerful crimson or " purple " to 

 the normal eye, will be grey to dichromic eyes, or, in other 

 words, a red invisible to them. And, similarly, in a line GU, 

 the point where it cuts the grey will be an " invisible green. " 



In this way, the figure, when completed for the various 



