407 



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 

 chamber 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 can be 

 altered in breadth and position, the position being 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 

 compartment of the instrument, and is reflected at a surface of 



