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III. The Threshold of Vision for Different Coloured Lights. 
Bij Sir W. de W. Abney, K.C.B., D.C.L ., D.Sc., F.B.S., and 
Prof. W. Watson, D.Sc., F.B.S. 
Received June 4,—Read June 17, 1915. 
1. The question as to the sensitiveness of the different regions of the retina to light 
of various wave-lengths is one which has received a considerable amount of attention. 
The results obtained by previous observers, while being in agreement in certain 
particulars, yet differ materially in others. For instance, the question as to whether 
dark adaptation takes place at the fovea and the controversy as to whether there is 
a failure of the secondary image at the fovea may be cited. In the course of some 
other experiments we were led to investigate certain differences which existed between 
our own eyes. The results obtained seem to throw a very considerable amount of light 
on the differences obtained by previous workers as well as to give interesting 
information as to the action of the retina. 
Our results support the idea that we have to do with two distinct processes when 
considering the variation in the sensations produced by light, a view which was first 
put forward by Parinaud and has been elaborated by yon Kries. The latter 
further identifies the rods and cones respectively as the two receptive elements of the 
retina. He supposes that the cones are responsible for all sensations of colour and 
are chiefly operative at medium and high illuminations. The rods he supposes are 
chiefly operative at low illuminations and only to come into action when the 
illumination is so low that all perception of colour is wanting, so that whatever the 
wave-length of the light stimulus the sensation produced is one corresponding to 
white or grey. Although our experiments do not provide any evidence as to the 
identification of the rods and cones with these two forms of vision, yet it saves so 
much circumlocution to speak of the sensation due to the cones or rods as the 
case may be that we shall use the terminology of yon Kries. Thus when we 
speak of the sensation due to the rods we must be understood to mean the 
sensation due to that mechanism, whatever it is, which is alone operative at low 
illuminations in the central part of the retina, and similarly for the sensation due to 
the cones. 
VOL. ccxyi.—a 540. 
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[Published December 6, 1915. 
