THE THEOET OF COMPOUND COLOUES. 
69 
Table II. (continued.) 
Scale. 
(N + w). 
Wave-length. 
Colour. 
64 
51-8 
1721 
Blue. 
68 
52-8 
1688 
Blue. 
72 
53-7 
1660 
Indigo. 
76 
54-7 
1630 
Indigo. 
80 
55-6 
1604 
Indigo. 
Ha-ving thus selected sixteen distinct points of the spectrum on which to operate, and 
determined their wave-lengths and apparent colours, I proceeded to ascertain the mathe- 
matical relations between these colours in order to lay them do^vn on Newton’s diagram. 
For this purpose I selected three of these as points of reference, namely, those at 24, 44, 
and 68 of the scale. I chose these points because they are well separated from each 
other on the scale, and because the colour of the spectrum at these points does not 
appear to the eye to vary very rapidly, either in hue or brightness, in passing from one 
point to another. Hence a small error of position will not make so serious an alteration 
of colour at these points, as if we had taken them at places of rapid variation ; and we 
may regard the amount of the illumination produced by the light entering through the 
slits in these positions as sensibly proportional to the breadth of the slits. 
(24) corresponds to a bright scarlet about one-third of the distance from C to D ; 
(44) is a green veiy near the line E; and (68) is a blue about one-third of the distance 
from F to G. 
§ YII. Method of Observation. 
% ^ 
The instrument is turned with the end AB towards a board, covered with white paper, 
and illuminated by sunlight. The operator sits at the end AB, to move the sliders, and 
adjust the slits ; and the observer sits at the end E, which is shaded from any bright 
light. The operator then places the slits so that their centres correspond to the three 
standard colours, and adjusts their breadths till the observer sees the prism illuminated 
with pure white light of the same intensity with that reflected by the mirror M. In 
order to do this, the observer must tell the operator what difference he observes in the 
two halves of the illuminated field, and the operator must alter the breadth of the slits 
accordingly, always keeping the centre of each slit at the proper point of the scale. The 
obsers'er may call for more or less red, blue or green ; and then the operator must 
increase or diminish the width of the slits X, Y, and Z respectively. If the variable field 
is darker or lighter than the constant field, the operator must widen or narrow all the 
slits in the same proportion. When the variable part of the field is nearly adjusted, it 
often happens that the constant white light from the mirror appears tinged with the 
complementary colour. This is an indication of what is required to make the resem- 
blance of the two parts of the field of Hew perfect. When no difference can be detected 
between the two parts of the field, either in colour or in brightness, the observer must 
look away for some time, to relieve the strain on the eye, and then look again. If the 
eye thus refreshed still judges the two parts of the field to be equal, the observation 
MDCCCLX. L 
