228 Intelligence and Miscellaneous Articles. 



to be this difference of potential " corrected for the effect of the 

 current passing (if appreciable)" the correction being the ad- 

 dition of the quantity bC= y E. This is obviously equiva- 

 lent to denning the E.M.F. as (V-V 7 )^^- 



I have the honour to be, Gentlemen, 



Your most obedient Servant, 



Charles E. Alder Wright. 



Chemical Laboratory, St. Mary's Hospital, W., 

 August 3, 1881. 



XXIX. Intelligence and Miscellaneous Articles. 



ON THE DETERMINATION OF THE ANGULAR DISTANCE OF THE 

 COLOURS. BY M. A. ROSENSTIEHL. 

 TWO series of experiments made upon a chromatic circle called my 

 ■*- attention to three colours appearing to fulfil special functions*. 

 I was thus led to discuss Young's theory t on the three fundamental 

 sensations, and to establish two categories of non- complementary 

 colours : — 1, those which by their mixture in threes at equal inten- 

 sity produce the sensation of white ; 2, those which produce that 

 sensation outside of those proportions. 



I have designated the first category by the name "triad;" and I 

 remarked that from their definition it follows that, representing 

 the table of colours by an equilateral triangle of which each vertex 

 is occupied by one of the colours of a triad, the complementaries 

 will be placed, in that construction, on the sides of the triangle and 

 at the extremities of a straight line passing through the point of 

 meeting of the median lines, while this regularity cannot exist for 

 the second category of combinations of three colours. 



The aim of the present note is to prove that the three colours 

 above referred to, viz. orange, the third yellow-green, and the third 

 blue, possess the characters of a triad. 



By a first series of experiments I determine the pairs of comple- 

 mentary colours. By a second series I ascertain the proportions in 

 which the three colours chosen must be mixed in order to obtain 

 the sensation of white. By a third series I seek to reproduce a 

 determined colour by the mixture of two or three colours : all 

 these experiments are made with rotating disks ; the measurement 

 of the angles of the sectors gives the position of this colour on the 

 sides of the triangle. In the calculations I do not take into consi- 

 deration the white which is produced at the same time, but only 

 the sensation of colour, my aim being to determine the proportion 

 in which two different colour-sensations must be mixed to obtain a 



* Phil. Mag. March and April 1881, vol xi. pp. 222, 305. 

 f Ibid. vol. xii. p. 78. 



