124 
SIR W. DE W. ABNEY AND PROF. W. WATSON ON 
rod and cone sensation which would increase the luminosity values in the blue and 
violet. 
Using the luminosity values given in Table IV. for rod vision, as deduced from 
experiments on monochromats, we have calculated the corresponding visibility curve 
and it is shown in fig. 25. 
6. Relative Sensitiveness of the Foveal and Parafoveal Regions. 
Observer W., with a rodless fovea, made a series of measurements of the relative 
intensities of two lights, one of which falls on the fovea and the other on the 
parafoveal region, when the sensations produced are the same, that is when the two 
lights appear equally bright. In this way we compare the sensitiveness of a part 
of the retina where there are only cones with that of a region where there are both 
rods and cones, and by making this comparison at different intensities of the stimulus 
light we obtain an insight as to the manner in which the relative sensitiveness of the 
rods and cones varies with the intensity. 
The arrangement employed is shown diagrammatically in fig. 26. The light of the 
required colour passes through a slit, A, attached to the slide of the spectrum 
apparatus, and a lens, B, is so adjusted as to give an image of the first face of the 
prism train on the screen, EF. The light passes through a sheet of plane glass, C, 
placed at 45 degrees, and the part of the light reflected from this glass is again 
reflected from a silvered mirror, D, on to the screen. This screen consists of a central 
white disc, F, of such a size that its radius subtends an angle of 43 minutes at the 
observer’s eye and a circular annulus, E, of which the radius of the inner edge subtends 
an angle of 2 degrees at the observer’s eye and that of the outer edge 3 degrees. The 
space between the central disc and the annulus is painted dead black. A diaphragm, 
G, is so arranged that the light which passes straight through the glass plate, C, only 
illuminates the disc, F ; while an opaque disc, H, screens this central disc from the light, 
which has been reflected from the mirrors C and D, so that this light only illuminates 
the annulus, E. A set of adjustable sectors, I, placed in the path of the light which 
