214 The Inheritance of Wing Colour in Lepidoptera 



be done, it should be remembered that one unit of bhie, and one unit of 

 red, and one unit of yellow when combined give one unit of black or 

 neutral tint : and, further, that one unit of red and one unit of yellow 

 together give one unit of orange. Now it is clear that in the above 

 example the eye will only receive 0*4 units of black, because not more 

 than 0*4 units can traverse all three colours. Since 0*4 units of red have 

 now been required to produce the neutral tint, only 5*2 units of red 

 remain. These, combined with an equal number of units of yellow, 

 give 5*2 units of orange. Now 0'4 + 5*2 units of yellow have been used 

 to produce the neutral tint and the orange, which when subtracted from 

 the total 9'8 units of yellow, leave 4*2 yellow units remaining. Thus 

 to obtain the visual colours from the glass units: treat the blue as 

 black; subtract this from the red to obtain the orange; and subtract 

 the red from the yellow to obtain the yellow. The two expressions thus 

 found may be considered virtually as the two halves of an equation. 

 As an example of the readings usually obtained the following colour 

 measurements of two insects are taken from the protocols : 



Throughout the paper all figures in square brackets refer to colour 

 measurements. The first number is the orange value, the second 



Or. Yel. 

 number the yellow value, thus : [2*5 : 2*2]. As a rule, the black value, 

 if any, was so small that it has been omitted. 



III. Method of presenting Results. 

 (a) Curves showing the distributions of the colour-values (Figs. 15 — 25). 



It was found that the colours of the insects did not fade appreciably 

 when they were kept in the dark. The colour of every insect was taken 

 separately in the manner already described (see p. 213), the conditions 

 of lighting, etc., being kept as constant as possible. With the readings 

 obtained from a series of insects, curves to show the distribution of the 

 colour- values can be constructed by plotting these values as ordinates, 

 the abscissae being determined by the numerical positions of the indi- 

 viduals in the series, after the colour- values of all the insects have been 



