124 
Proceedings of the Royal Society 
over these holes coloured transparent media of any kind, or direct 
them towards coloured surfaces, then on looking through the lens 
all the colours will be combined and we shall see the resultant 
colour. 
The following is a short description of an instrument, which I 
have constructed on this principle, and used with satisfactory 
results. It consists of a long wooden box about 70 cm. long by 
12 cm. deep and 12 cm. wide. In one end of the box are two 
round openings, placed a short distance apart, one over the other. 
The lower opening has a diameter of about 5 cm., and has three 
radial bars fixed in it, dividing the circle into three equal sectors. 
Into these divisions are fitted pieces of glass of different colours. 
What these three colours are I shall presently state. The upper 
opening is of a less diameter than the lower, and is covered with 
clear glass ; the purpose of this opening will be referred to presently. 
Each of the sectors in the lower opening is provided with a 
shutter to regulate the amount of light passing through it. The 
upper opening is also provided with a movable shutter. Into the 
opposite end of the box are fitted two eye-pieces, so that, if desired, 
observations can be made with both eyes at the same time. The 
lenses of the eye-pieces are similar to a Ramsden eye-piece, and 
each consists of two plano-convex lenses, mounted at one end of a 
tube about 30 cm. long. These tubes slide in sockets fixed in the 
end of the box. The shutters for regulating the quantity of light 
entering the apparatus are each mounted on the end of a horizontal 
shaft, which is pivoted at the front end of the box, and passes 
back to the end next the eye-pieces, where levers are fixed to the 
end of each, so that each shutter can be moved separately. By 
this means the amount of the different colours may be conveniently 
regulated by the observer, while his eye is applied to the instru- 
ment. For some observations an opening of 5 cm. for admitting 
the different coloured lights is too large, as it separates the colours 
too far ; stops of different sizes are therefore provided for reducing 
this opening to the size necessary to suit the experiment. 
The colours produced by this instrument are brilliant and beauti- 
ful, reminding one of the varied and delicate colours produced by 
selenite plates and polarised light, and by it we can easily combine 
the colours so as to give white. 
