IGO 
CAPTAIN ABNEY ON THE SENSITIVENESS 
It may be noted that the annulus was graduated at the same distance from the 
screen as that at which it was employed in the subsequent experiments. This is, 
perhaps, an unnecessary refinement in the case of a medium presenting such fine 
particles, though it Avould not have been so had they been as coarse as the grains of 
sdver in a photographic negative, for in the last case a certain amount of light is lost 
through scattering. The annulus having been graduated, it became necessary to 
ascertain what breadth of annulus might be used so that no appreciable error might 
be incurred by reading the centre of the slit as the mean reading of the light passing 
through it. If the slit occupied 10° of the annulus, the light passing through the 
marcrins would be as 2051 to 1683, and the mean of these two would be 1867. The 
light passing through the centre of the slit would be in reality 1858 on the same 
scale. There would be, therefore, a difference between the mean of the marginal 
rays and that of the mean absorption of ’9 per cent. 
If the breadth of slit occupied 5° of the annulus, the difference between the mean 
of the marginal intensities and that of the actual central intensity would be '2 per 
cent. The true mean of all the rays would be less than these figures. A slit of 5° 
width would, therefore, be admissible to use as giving an error much less than that 
found in observations—as a matter of fact, the slit was always considerably narrower 
than this, so that no appreciable error can be found on this account. 
4. New Extinction Box. 
In Part III. of “Colour Photometry ” a diagram is given of an “extinction box,” 
and also of the curves of extinction of the visible spectrum for various classes of 
colour vision. For the greater part of the experiments the box was slightly altered, 
so far as regards the method of admitting the light. 
Fig. 3. 
At the end of the box, B, an aperture was cut, which was closed by a piece ot 
glass, G, finely ground on each side, or by an opal glass. Provision was made lor 
