66 
PEOFESSOE J. CLEEK MAXWELL OX 
Now suppose light to enter at E, to pass through the lens, and to be refracted by the 
two prisms at P ; a pure spectrum, showing FEArxHOFEE’s lines, is formed at AB, but 
only that part is allowed to pass which falls on the three slits XYZ. The rest is stopped 
by the shutters. Suppose that the portion falling on X belongs to the red part of the 
spectrum ; then, of the white light entering at E, only the red will come through the 
slit X. If we were to admit red light at X it would be refracted to E, by the principle 
in Optics, that the course of any ray may be reversed. If, instead of red light, we were 
to admit white light at X, still only red light would come to E ; for all other light would 
be either more or less refracted, and would not reach the slit at E. Apphung the eye 
at the slit E, we should see the prism P uniformly illuminated with red light, of the kind 
corresponding to the part of the spectrum which falls on the slit X when white light is 
admitted at E. 
Let the slit Y correspond to another portion of the spectrum, say the green ; then, if 
white light is admitted at Y, the prism, as seen by an eye at E, ^vill be uniformly illu- 
minated with green light ; and if white light be admitted at X and Y simultaneously, 
the colour seen at E will be a compound of red and green, the proportions depending 
on the breadth of the slits and the intensity of the light which enters them. The thii’d 
slit, Z, enables us to combine any three kinds of light in any given proportions, so that 
an eye at E shall see the face of the prism at P uniformly illuminated with the colour 
resulting from the combination of the three. The position of these three rays in the 
spectrum is found by admitting the light at E, and comparing the position of the slits 
with the position of the principal fixed lines ; and the breadth of the slits is determined 
by means of the wedge. 
At the same time white light is admitted through BC to the mirror of black glass at 
M, whence it is reflected to E, past the edge of the prism at P, so that the eye at E sees 
through the lens a field consisting of two portions, separated by the edge of the prism ; 
that on the left hand being compounded of three colours of the spectrum refracted by 
the prism, while that on the right hand is white light reflected from the muTor. By 
adjusting the slits properly, these two portions of the field may be made equal, both m 
colour and brightness, so that the edge of the prism becomes almost imisible. 
In making experiments, the instrument was placed on a table in a room moderately 
lighted, with the end AB turned towards a large board covered nith white paper, and 
placed in the open air, so as to be uniformly illuminated by the sun. In this way the 
three slits and the mirror M were all illuminated with white light of the same intensity, 
and all were affected in the same ratio by any change of illumination ; so that if the 
two halves of the field were rendered equal when the sun was under a cloud, they were 
found nearly correct when the sun again appeared. No experiments, however, were 
considered good unless the sun remained uniformly bright during the whole series of 
experiments. 
After each set of experiments light was admitted at E, and the position of the fixed 
lines D and F of the spectrum was read off on the scale at AB. It was found that after 
