536 
CAPTAIN W. DE W. ABNEY AND MAJOR-GENERAL E. R. RESTING 
Fig. 34. 
Absorption by tbe macula luiea. 
§ XLVI .—The Fovea Centralist 
The question of visual sensation^at the fovea centralis, if that be admitted to be 
coincident with the visual axis of the eye, has occupied our attention, and we have 
thought it worth while to give the measures of luminosity when the images of the 
illuminated shadows fell on this portion of the retina. A cube of ^-inch side was 
prepared, and the beams of light allowed to fall on it in the usual manner. The 
luminosities of the white and coloured shadows were equalised when they were 
observed at a distance of 60 inches from the eye. One eye was closed during 
the observations. The measures made are given in Table I., Col. Y. It will be 
noticed the fovea is rather more sensitive to the red rays than the macula lutea, and 
is in general much less sensitive to the green rays. A calculation of the areas of the 
curves of luminosity shows that the fovea is ^ more sensitive to D light than the 
macula lucea as a whole. It is somewhat remarkable that the sensitiveness to green 
and blue of the fovea is not greater, and is even less at certain places, than of the 
macula lutea, considering the almost entire absence of pigment from the former. 
If the small cube be examined at still further distances there is a still further 
increase in the luminosity of the red and a further decrease in that of the green. 
Ydiat the limit may be where no further change takes place we are not at present 
prepared to say. If a star or a distant light be observed in the point where the 
visual axis of the eye cuts the retina, and then on the part of the retina slightly 
removed from this point, the different colour of the images will be evident. 
* Added July 20. 
