ON COLOUR PHOTOMETRY. 
541 
The necessity of resting the eye for some time in darkness in order to give it the 
full sensitiveness to feeble light was soon recognised. When extinction has been 
made and the instrument left untouched, if the eye was exposed to the light of day 
for some time, and then an observation was made, even after two minutes rest, no light 
from any ray of the spectrum was visible in the extinction box, even with the sectors 
removed ; after a further rest of two minutes the rays last to be extinguished could 
be perceived ; and finally, after about ten minutes’ rest, the eye became of the same 
sensitiveness as before it was exposed. When several successive extinction readings 
of the same ray agreed, it was considered that the eye was in a fit state to commence 
a series of observations. 
The apparatus used was usually of the form described below, but variations in its 
arrangement and in the methods of observations were made from time to time, in 
order to track out any possible error. 
Fig. 38. 
B B 
Apparatus to measure extinction of light. 
BB (fig. 38), is a closed box 3 feet long and about 1 foot wide and 1 foot high, 
having two circular apertures 1 \ inches in diameter in the positions shown. The 
aperture at the side is covered on the inside by a piece of glass, a, finely ground on 
both sides, and a tube, T , is inserted in which diaphragms, D, of any required 
aperture can be inserted. E is a tube fixed into the other aperture, and should 
for comfort be fitted with an end shaped to receive the eye, as the observations are 
made through it. S is a cardboard screen inserted from the top of the box, the 
