THE THRESHOLD OF VISION FOR DIFFERENT COLOURED LIGHTS. 
I 19 
measurements in this region are more uncertain than in the brighter parts of the 
spectrum. Differences in the absorption values of the eye-media are also most likely 
to have the greatest effect in this region. 
. 4. Method of Examining the Distribution of Bods in the Retina. 
The determination of the threshold values for foveal fixation, as described above, 
enables us at once to say whether an observer belongs to class A. or to class B. 
Since, however, it takes a considerable time to obtain a set of observations, as it is 
necessary to go on taking sets till it is quite certain that correct foveal fixation is 
being obtained, examining any number of persons by this method is a very lengthy 
process. Hence we have devised another method which enables one in a few minutes 
to settle to which class an observer belone’s. 
c~) 
By means of a plane mirror, M, fig. 23, to which is imparted an oscillatory motion 
about a vertical axis, the light passing through the slit, C, attached to a slide, AB, 
which is in the plane of the spectrum, is reflected on to a white screen, HJ. By 
means of a diaphragm placed against the first face of the prism train of the colour 
