170 
CAPTAIN ABNEY ON THE SENSITIVENESS 
same relation to the abscissae that they did in case of circular apertures. We may 
therefore presume, with a considerable degree of certainty, that it is the width of the 
aperture, horizontal or vertical (for the experiments were repeated in both directions), 
that governs the extinction. These results, it will at once be seen, have an important 
bearing on spectroscopic work, and the invisibility of lines when the light is feeble. 
Fig. fi. 
Taking into consideration the extinction curve of the spectrum, and these results, 
we can see how the green lines of a feeble spectrum will be the first to be seen 
(perhaps colourless), whilst others, though present, will fail to be seen except with 
a very wide slit. 
A further experiment was made which confirmed the previous measures. The 
extinction of the light from a circular, a square, and a rectangular aperture of the 
same area was made. The circular aperture had a diameter *94 inch, the sides of 
the square were *84 inch, and of the oblong 1*68 X ‘42 inch. In addition, an oblong 
aperture *84 X *42, exactly half the latter, was also used. 
The following are the results of the extinction, and in the last column are given 
the results that would have been obtained from the curves already described:— 
