396 HERMANN VON HELMHOLTZ 



believes that such an attempt must be made, in order to obtain 

 the preliminary orientations in a new department. 



If the brightness of two somewhat differently coloured lights 

 be compared together, a point is arrived at with gradual 

 alteration of the intensity of one of them, at which the per- 

 ceptible colour difference reaches a minimum of clearness; 

 the ratio of light intensities corresponding to this point is then 

 regarded as the ratio of equal brightness. Helmholtz next 

 proposed to himself the task of ascertaining this point of least 

 recognizable difference for a series of mixed colours, obtained 

 from the same colour-elements by admixture upon the colour 

 disc. He found in the first place that the effect of an increment 

 of any colour upon the brightness was essentially diminished 

 by the amount of that same colour already present in the 

 mixture ; it follows from the experiments that if, starting from 

 a highly saturated colour, we determine a series of mixed 

 colours of equal brightness (by always comparing two very 

 closely connected members of the series with one another), 

 the total quantity of mixed light in such a series of colours 

 cannot remain unaltered. If we begin with the most highly 

 saturated red, we shall diminish the brightness far less by 

 subtracting a small quantity of red than we strengthen it by 

 adding an equal quantity of blue. The comparison thus effected 

 between two approximately equal and highly saturated colours 

 is essentially different from the case in which the brightness 

 of two very differently coloured fields is compared together. 

 A long series of experiments led in the first place to the result 

 that the recognizability of low gradations of the intensity of 

 coloured light is far less affected by the simultaneous presence 

 of a second and quite dissimilar colour in the field, than it is 

 by the presence of an equally bright quantity of the same colour. 



The extension of the form of the psycho-physical law to 

 complexes of more than one dimension had led Helmholtz 

 (while Fechner's law referred merely to alterations in light 

 intensity with unaltered mixture of the light) to the quantitative 

 determination of the nature of a colour-sensation in dichromatic 

 eyes by two independent variables, in trichromatic eyes by 

 three variables. In these two papers (one published with the 

 title 'Attempt to apply the Psycho-physical Law to the 

 Differences of Colour in Trichromatic Eyes/ in the Zeitschr. 



