THE THEOET OE COMPOUND COLOUES. 
65 
intensity of white. In this way the effects of subjective colours were entirely got rid of, 
and all the observations were of the same kind, and therefore may claim to be equally 
accurate ; which is not the case when comparisons are made between bright colours of 
different kinds. 
The chart of the spectrum, deduced from these observations, exhibits the colours 
arranged very exactly along two sides of a triangle, the extreme red and violet forming 
doubtful portions of the third side. This result greatly simplifies the theory of colour, 
if it does not actually point out the three primary colours themselves. 
§ V. Description of an Instrument for making definite Mixtures of the Colours of the 
Spectrum. 
The experimental method which I have used consists in forming a combination of 
three colours belonging to different portions of the spectrum, the quantity of each being 
so adjusted that the mixture shall be white, and equal in intensity to a given white. 
Fig. 1, Plate I. represents the instrument for making the observations. It consists of 
two tubes, or long boxes, of deal, of rectangular section, joined together at an angle of 
about 100°. 
The part AK is about 5 feet long, 7 inches broad, and 4 deep ; KN is about 2 feet 
long, 6 inches broad, and 4 deep ; BD is a partition parallel to the side of the long box. 
The whole of the inside of the instrument is painted black, and the only openings are at 
the end AC, and at E. At the angle there is a lid, which is opened when the optical 
parts have to be adjusted or cleaned. 
At E is a fine vertical slit ; L is a lens ; at P there are two equilateral prisms. The 
slit E, the lens L, and the prisms P are so adjusted, that when light is admitted at E a 
piu'e spectrum is formed at AB, the extremity of the long box. A mirror at M is also 
adjusted so as to reflect the light from E along the narrow compartment of the long box 
to BC. See fig. 3. 
At AB is placed the contrivance shown in fig. 2, Plate I. A'B' is a rectangular frame 
of brass, ha\ing a rectangular aperture of 6 X 1 inches. On this frame are placed six 
brass sliders, XYZ. Each of these carries a knife-edge of brass in the plane of the sur- 
face of the frame. 
These six moveable knife-edges form three slits, XYZ, whicn may be so adjusted as 
to coincide with any three portions of the pure spectrum formed by light from E. The 
intervals behind the sliders are closed by hinged shutters, which allow the sliders to 
move without letting light pass between them. 
The inner edge of the brass frame is graduated to twentieths of an inch, so that the 
position of any slit can be read off. The breadth of the slit is ascertained by means of 
a wedge-shaped piece of metal, G inches long, and tapering to a point from a breadth of 
half an inch. This is gently inserted into each slit, and the breadth is determined by 
the distance to which it enters, the divisions on the wedge corresponding to the 200th 
of an inch difference in breadth, so that the unit of breadth is '005 inch. 
