: j COLOUR M8IOX. 



This is only one of many ways of stating the position of a point, but 

 it is one of the most convenient. Now, colour also depends on three things. 

 If we call these the intensities of the three primary colour sensations, and it' 

 we are able in any way to measure these three intensities, we may consider 

 the colour as specified by these three measurements. Hence the specification of 

 a colour agrees with the specification of a point in the room in depending on 

 three measurements. 



Let us go a step farther and suppose the colour sensations measured on 

 some scale of intensity, and a point found for which the three distances, or 

 co-ordinates, contain the same number of feet as the sensations contain dcL 

 of intensity. Then we may say, by a useful geometrical convention, that the 

 colour is represented, to our mathematical imagination, by the point so found 

 in the room; and if there are several colours, represented by several points, the 

 chromatic relations of the colours will be represented by the geometrical rela- 

 tions of the points. This method of expressing the relations of colours is a 

 great help to the imagination. You will find these relations of colours s* 

 in an exceedingly clear manner in Mr Benson's Manual of Colour one of the 

 very few books on colour in which the statements are founded on legitimate 

 experiments. 



There is a still more convenient method of representing the relations of 

 colours by means of Young's triangle of colours. It is impossible to repn 

 on a plane piece of paper every conceivable colour, to do this requires space of 

 three dimensions. If, however, we consider only colours of the same shade 

 that is, colours in which the sum of the intensities of the three sensations is 

 the same, then the variations in tint and in hue of all such colours may be 

 represented by points on a plane. For this purpose we must draw a plane 

 cutting off equal lengths from the three lines representing the primary sensat 

 The part of this plane within the space in which we have been distributing our 

 colours will be an equilateral triangle. The three primary colours will be at the 

 three angles, white or gray will be in the middle, the tint or degree of purity 

 of any colour will be expressed by its distance from the middle point, and its 

 hue will depend on the angular position of the line which joins it with the 

 middle point. 



Thus the ideas of tint and hue can be expressed geometrically on You 

 triangle. To understand what is meant by shade we have only to suppose the 

 illumination of the whole triangle increased or diminished, so that by means of 



