128 
Proceedings of the Royal Society 
light of each colour entering and falling on the white paper at 
the bottom of this compartment. An opening is made in one side 
of the box, and arrangements made, so that while light is excluded, 
the observer can look down on the bottom of both compartments 
of the box, and observe the colour of the illumination on the paper. 
When one eye is placed directly in front of the thin metal partition 
it can see the bottom of both compartments at once, divided by the 
thin line of the partition. The paper in the one compartment, 
lighted by white light, is thus only separated by a fine line, from 
the other one illuminated by the coloured lights, and comparison is 
thus easily made. When working with this colour box it is taken 
outside or placed at an angle of about 45 degrees at an open window. 
By opening the shutters over the coloured glasses to the proper 
degree, the paper on the bottom of this compartment may be made 
to appear of any colour or tint we may desire. The other compart- 
ment of the box is illuminated with white light, and the shutter is 
opened to the desired amount to provide the eye with a standard of 
white. 
When the three colours are combined in the proper proportions, 
so perfect a white can be produced with this apparatus that it is 
impossible to distinguish between the standard white and the white 
produced by the three brilliant colours. In order to get this 
perfect match of the two whites, great care must be taken in the 
construction of the apparatus. The surface of the interior of the 
box must be dead white ; if there is the smallest amount of polished 
surface reflection, such as that given by most paints, then the 
field of observation will not be flat, but will be differently tinted at 
different points. If the surfaces in the interior were perfectly flat, 
these reflections would not interfere with the result; but, owing to 
the unavoidable curvatures of these surfaces, the colours are 
unequally reflected to the paper. The source of the illumination 
required for working this box ought to have as wide an angle 
as possible. A bright sky does very well, but if the sun shines 
on the instrument, ground glass or thin white paper must be hung 
up in front of the glasses. 
One advantage of this colour box is the ease to the eye with 
which the observations can be made, and the secondary colours, 
such as yellow, blue, and purple, produced by it are brilliant. If we 
