OF THE RETINA TO LIGHT AND COLOUR. 
173 
coming through the slit is. therefore, diminished till the two appear equally bright. 
The diminution of light is noted, that coming through the larger aperture being 
diminished twice as much as that coming through the smaller. The sectors are again 
set at 45°, and the same procedure adopted as before. 
Table X. gives the measurements thus made, and fig. 7 shows them diagrammati- 
cally. The ordinates are the intensities of light of the large aperture. These are 
derived from the value of diminution of the light falling on the ground glass, together 
with the reduction due to the sectors. The latter is converted into the degrees of the 
annulus and added to that by which the light has been diminished before falling on the 
plate. The abscisste show the values of the reduction of the light on the smaller 
aperture. Both the annular values are shown as logarithms. Again, the resulting 
curve is for a large part of its length a straight line. Each aperture has its own 
inclination and is determined by the extinction values of the two apertures. In making 
these determinations the eye has to judge the brightness of very dissimilar sizes of area, 
and it might be thought that this fact would present an almost insuperable difficulty in 
making very accurate measures. As a matter of fact, it was not so j the greatest 
difficulty was encountered in those cases when the light of the large aperture was so 
diminished that it became colourless, whilst the other had very nearly its original tint. 
The red was perhaps the hardest to judg'e on that account; the other colours did not pre- 
sent any great difficulty. One of the curious phenomena encountered in these measures 
at times was a distinct scintillation of the light emitted by the small aperture. Some¬ 
times this was perplexing, but never to the extent to render the comparisons uncertain. 
