192 
CAPTAIN ABNEY ON THE SENSITIVENESS 
19. Relative Sensitiveness of the Different Parts of the Retina. 
One other determination of sensitiveness of the retina required to be made, viz., 
the general sensitiveness at all parts compared with that at the centre or close 
to the yellow spot. In “ Colour Photometry,” Part III., a comparison was made 
of the sensitiveness of the centre of the eye compared with that of a point 10° 
towards the periphery. Determinations of this kind are extremely difficult, and it 
is only by continued observation that an approach to correct measures can be made. 
In fact the eye requires training. Perhaps the easiest plan of explaining how the 
following determinations were made will be by describing a preliminary experiment. 
Procure a large sheet of black paper and lay it horizontally on a table near a window, 
so that it is equally illuminated. Cut out some small and equal discs of white paper 
or card and place two of them about 1 foot apart lying on the black paper. Place 
the eye about 12 inches above one of them, and receive its image on the centre 
of the retina. At the same time the image of the other will be received on the 
retina about 45° from the centre. This last white disc will appear to be very 
decidedly darker than the first. 
Cut out a small disc in grey paper, and substitute it for the white disc, the image of 
which is viewed centrally. The other white disc may now be moved away from it till 
the two appear equally luminous. The distance from the grey disc to the white will 
give the field. By measuring the amount of white light reflected from the grey 
paper, the comparative luminosities of the discs are found, and from them the relative 
sensitiveness of the two portions of the retina are determined. 
The same procedure can be carried out on an equally illuminated surface, and “ iso- 
lumes ” be made for any depth of grey. The following table gives one of the deter¬ 
minations, the grey in this case reflecting of the white light reflected from the 
white disc. The diameters of the discs were half an inch, and were viewed with the 
right eye at a distance of two feet from a vertical screen (see fig. 18). 
Table XXIII. 
Angle with the 
vertical. 
Field, 
in degrees. 
Angle with the 
vertical. 
Field, 
in degrees. 
0 
24 
180 
26 
30^ 
27 
1501 
25 
60 
33 
120 
27 
90 
^N. 
39 
90 
^T. 
30 
T20 
40 
60 
27 
150 J 
30 
30j 
25 , 
It will be seen that the “ iso-lumes ” are of the same character as the colour fields. 
Some small correction might have to be made for the projection of the white disc 
on the retina, since it would not be of the same angular dimensions as if viewed in a 
hemispherical perimeter. 
