MIXTURE. 1 08 1 



probably depend chiefly on the method of the investigator. It is 

 generally put at y^ of the total stimulus, but the delicacy of dis- 

 crimination appears to be greater, the figure given by Helmholtz being 

 j-^- ; by Aubert, T ^ ; by Schirmer, -^^ ; and by Ole Bull, 0-^5-. In some 

 individuals, the delicacy of discrimination is even higher than this. 



Aubert, 1 using the shadow method, found that as the illumination 

 diminished the threshold of discrimination increased ; and Helmholtz, 2 

 using Masson's discs, found a similar departure from Weber's law at low 

 luminosities. Krapelin 3 and Schirmer, 4 on the other hand, found that 

 the threshold of discrimination is constant when due attention is paid to 

 adaptation, and that, with maximal adaptation to the illumination at 

 which the observations are made, Weber's law is true within wide limits. 

 Krapelin found that the results were uninfluenced by the size of the 

 pupil. 



MlXTUEE. 



Colour mixture. — The mixture which occurs when an area of the 

 retina is simultaneously stimulated by different kinds of light, may be 

 effected in several ways. In an old method, due to Lambert, a coloured 

 patch is placed on each side of a plate of glass held vertically, so that one 

 patch is seen by reflected, the other by transmitted, light. The relative 

 intensities of the two components may be varied by varying the angle 

 at which the line of vision cuts the glass. 



Mixture due to double refraction may be seen if two patches are 

 observed through a crystal of Iceland spar, so that two of the four 

 patches produced coincide. 



In pigment colours the most usual method is by means of the colour 

 wheel, — a method depending upon visual persistence. Spectral colours 

 may be allowed directly to combine on a retinal area, or may be con- 

 verged by means of a lens on a screen. 



The colour triangle.— The representation of the facts of colour mix- 

 ture by means of a geometrical diagram is due to Newton. He adopted 

 a circle with its circumference divided into parts corresponding to the 

 colours of the spectrum. At the centre of gravity of each of the seg- 

 ments corresponding to these parts, he g 

 placed a circle, with its area propor- 

 tional to the amount of light of each 

 colour entering a given mixture. The 

 point in the diagram representing the 

 centre of gravity of all these circles gives 

 the nature of the mixture. A radius 

 drawn through it gives the hue, and the 

 distance of the point from the centre 

 the degree of saturation. 



This method of calculating the FlG 391# 



results of colour mixture was verified 



experimentally by Maxwell. In a colour diagram each point represents 

 a colour. The resultant of a mixture of any two colours is found by 

 dividing the line joining the two points of the diagram inversely as the 

 quantity of each ; thus the resultant of mixture of two parts of red with 



1 " Physiol, d. Netzhaut," S. 52. 



2 " Handbuch d. physiol. Optik," 1867, S. 315. 

 3 Phil. Stud., Leipzig, 1885, Bd. ii. S. 306. 



4 Arch. f. Ophth., 1890, Bd. xxxvi. Abth. 3, S. 121. 



