ON COLOUR PHOTOMETRY. 
543 
appeared as a white oblong, though B was translucent and A opaque. The re-combined 
beam was then allowed to fall on the back of the card, and B became illuminated. 
On the other side of the card an amyl-acetate lamp illuminated A, B being screened 
by a rod in the path of the light. The brightness of A and B were made equal by 
placing the rotating sectors in the path of the beam of the amyl lamp, and thus the 
proportion of light passing through the paper was measured in terms of that reflected 
from a white surface. The card then replaced the screen S in the box, the end of the 
box was opened, and A and B were exposed to view. The light of the re-combined 
beam was directed on to T so as to illuminate S. A was made equally bright by the 
amyl-acetate lamp and the illumination calculated ; knowing from the former observa¬ 
tion what proportion of the light falling on B is visible from the other side, the 
amount of light falling on the screen, and, therefore, its proportion to that received at 
T could be determined. Measurements were taken with each diaphragm, and the 
illumination of the screen in terms of the light received at T, was found to be 
proportional to the areas of the apertures, as might be expected, and, as follows, for 
the diaphragms used :— 
No. 0 , -§ 7 ) j No. 1 , j 5 5 ; No. 2 , ; No. 3, ; No. 4, No. 5, ; No. 6 , 
_JL_ • No 7 _1_* 
9 5 6 ’ u - •> 2430 * 
The method of diminishing the illumination of the screen by ground-glass was 
found to be most effective. A beam of monochromatic light from the brightest part 
of the spectrum can be diminished to such an extent as to come within the limits of 
extinction by the rotating sectors, with the apertures of such an angular dimension 
as to be properly read (say more than 6°). 
The D light coming through the spectrum slit was measured against an amyl lamp 
by placing a white opaque screen at the aperture a (the tube T being removed). 
The luminosity of the D light being thus known, that of any other ray could be 
calculated from the curve A in fig. 33. Another method of observation was as 
follows : a diaphragm with a small circular aperture was placed in front of the last 
prism of the apparatus. The patch of light on the screen was now a small circular 
disc, instead of being square, as before. A similar box was prepared to that of fig. 38, 
but the ground-glass was omitted. The ray of light now falling on M formed a 
circular patch on the screen S, but the beam of light so formed is too powerful to be 
extinguished by any readable aperture of the rotating sectors, it was therefore 
further reduced by placing in its path, and at an angle of 45° to it, two parallel 
mirrors A, B (see fig. 40). Each mirror can be either silvered or plain glass ; three 
combinations of different reducing powers are therefore possible, viz. : (a) both mirrors 
silvered, ( 6 ) one plain and one silvered, (c) both plain. 
The proportion of the light reflected with each combination can be readily deter* 
* [This method of measuring the ratio of light falling on the screen to that on the ground glass has 
been subsequently modified, the two being directly compared one with the other.—July 20.] 
