Prof. Maxwell on the Theory of Compound Colours. 148 
Newton’s diagram. In this way any colour may be represented by 
a line drawn from the origin whose direction indicates the quality of 
the colour, and whose length depends upon its intensity. The 
resultant of two colours is represented by the diagonal of the paral- 
lelogram formed on the lines representing the colours (see Prof. 
Grassmann in Phil. Mag. April 1854). 
Taking three lines drawn from the origin through the points of 
the diagram corresponding to the three primaries as the axes of 
coordinates, we may express any colour as the resultant of definite 
quantities of each of the three primaries, and the three elements of 
colour will then be represented by the three dimensions of space. 
The experiments, the results of which are now before the Society, 
were undertaken in order to ascertain the exact relations of the 
colours of the spectrum as seen by a normal eye, and to lay down 
these relations on Newton’s diagram. The method consisted in 
selecting three colours from the spectrum, and mixing these in such 
proportions as to be identical in colour and brightness with a constant 
white light, Having assumed three standard colours, and found the 
quantity of each required to produce the given white, we then find 
the quantities of two of these combined with a fourth colour which 
will produce the same white. We thus obtain a relation between the 
three standards and the fourth colour, which enables us to lay down 
its position in Newton’s diagram with reference to the three standards. 
Any three sufficiently different colours may be chosen as standards, 
and any three points may be assumed as their positions on the 
diagram. The resulting diagram of relations of colour will differ 
according to the way in which we begin; but as every colour-diagram 
is a perspective projection of any other, it is easy to compare diagrams 
obtained by two different methods. 
The instrument employed in these experiments consisted of a dark 
’ ehamber about 5 feet long, 9 inches broad, and 4 deep, joined to 
another 2 feet long at an angle of about 100°. If light is admitted 
at a narrow slit at the end of the shorter chamber, it falls on a lens 
and is refracted through two prisms in succession, so as to form a 
pure spectrum at the end of the long chamber. Here there is 
placed an apparatus consisting of three moveable slits, which ean be 
altered in breadth and position, the position beimg read off on a 
graduated scale, and the breadth ascertained by inserting a fine 
graduated wedge into the slit till it touches both sides. 
When white light is admitted at the shorter end, light of three 
different kinds is refracted to these three slits. When white light is 
admitted at the three slits, light of these three kinds in combination 
is seen by an eye placed at the slit in the shorter arm of the instru- 
ment. By altering the three slits, the colour of this compound light 
may be changed at pleasure. 
The white light employed was that of a sheet of white paper, placed 
on a board, and illuminated by the sun’s light in the open air; the 
instrument being in a room, and the light moderated where the 
observer sits. 
Another portion of the same white light goes down a separate 
