Intelligence and Miscellaneous Articles. 79 



equal intensity and emitting, under equal illumination, the same 

 amount of white light. The circle with which I have worked does 

 not possess these qualities ; and I have renounced taking this course ; 

 moreover the difficulty can be easily turned. 



2. The sensation of pure white results from the equal excitation of 

 the three fundamental sensations. 



Each of these colours has for its complement the mixture of the 

 other two at equal intensity. 



The graphic construction representing the whole of the ^percep- 

 tible colours usually takes the form of an equilateral triangle, the 

 vertices of which are occupied by the primary colours. The corre- 

 sponding complementary, which is always a binary colour, is placed 

 in the middle of the opposite side. All the other binary colours 

 have their places also on the sides of the triangle, so that the two 

 complementaries are situated at the two extremities of a straight 

 line passing through the point of intersection of the median ones. 



The properties above enunciated, and the consequences I have 

 enumerated, and w 7 hich are admitted, are not characteristic of 

 the primary colours ; they belong to an indefinite number of colours, 

 on the condition, however, that they be selected according to a cer- 

 tain rule. An assemblage of three colours possessing this property 

 I call a triad. In the equilateral triangle above described the 

 secondary triads will he re-presented by all the equilateral triangles 

 which can be inscribed in it. The smallest of these triangles is that 

 formed by the triad of the complementaries of the primary colours. 

 To each secondary triad corresponds in like manner a triad com- 

 posed of complementary colours ; and as we can choose them as 

 near together as we please, there is really no limit to their number. 

 The distribution of the complementary colours in a triad is suscep- 

 tible of experimental verification ; and I shall have occasion to revert 

 to it. 



It is expedient to add, as a corollary, that three colours not con- 

 stituting a triad may still by their mixture produce the sensation 

 of white ; but this sensation no longer results from their mixture 

 in equal quantities. It follows that, in the graphic construction, 

 their complements are then not situated in the middle of the oppo- 

 site side. This point, like the preceding, has been verified experi- 

 mentally. 



I now 7 come to an essential character. If, in an equilateral 

 triangle, each vertex be joined to the middle of the opposite side, it 

 will be divided into six equal right-angled triangles. The colours 

 included in two of these triangles with their vertices opposed, are 

 reciprocally complementary. 



Given the distance b of a binary colour at the vertex of the equi- 

 lateral triangle, the position of the complementary colour will be 

 given by the distance x which separates it from the corresponding 

 vertex, w T hich distance is represented by 



sin a 



siu(30° + a) 



