III. The threshold of vision for different coloured lights

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Abstract
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 von 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 von 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.

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