ON COLOUR PHOTOMETRY. 
425 
the candle or lamp, T, for the comparison-light, moved on a lath of wood, to which 
was attached a scale commencing at the screen. When a patch of light of any- 
desired colour was thrown on the white screen the comparison light could be moved 
along the scale till the luminosities appeared to be equal. Very fair results were 
obtained by this plan, but the matter of reading was still somewhat difficult until 
what we have called the “ oscillation ” plan was hit upon. This plan we will now 
describe. 
Fig. I. 
The illuminating value of the spectrum varies greatly in its different parts; the 
maximum is usually placed in the orange or yellow, and there is a gradation towards 
each end of the visible spectrum, the rapidity of which varies according to the kind of 
light used and the part of the spectrum examined. Now suppose we find that with 
a source of light at 24 inches from the screen it is approximately of the same intensity 
as the yellow ray of the spectrum, it is manifest that with the source at, say, 30 inches 
from the screen, its light will be balanced by that of either of two portions of the 
spectrum, one on the red the other on the blue side of the yellow. To ascertain 
which those portions are, the slit A is first moved gently from the yellow to the green- 
blue. When in the yellow the shadow of the rod illuminated by the source of white light 
will be palpably darker than the other, and when the slit has passed into the green- 
blue it will be palpably lighter. We find that the best way of determining the 
intermediate point where the shadows balance is by oscillating the slide gently 
between points where first one shadow and then the other is palpably too dark ; the 
oscillations become shorter and shorter until the point of balance is determined. The 
slide is then moved from the yellow through the red and the same process repeated. 
We thus get the two points in the spectrum whose illumination corresponds to that of 
the source of light at its then distance from the screen. By successive alterations in the 
distance of the comparison-light other pairs of points in the spectrum are determined 
until the limits of the visible spectrum are reached. The curve of intensities of 
different parts of the spectrum plotted from these observations will be found to be 
fairly smooth. This curve we call the “ luminosity curve.” 
MDCCCLXXXVI. 3 I 
