For the sunfish Enneacanthus the mean value of the critical illumination for response to visual flicker at constant flash frequency (with light time = dark time) is related to temperature by the Arrhenius equation. The temperature characteristic for 1/Im is different above and below 20°C. In each range (12° to 20°; 20° to 30°) the temperature characteristic is the same for rod and cone segments of the duplex flicker response contour: 8,200 and 14,400. This makes it difficult, if not impossible, to consider that the two groups of elements are organized in a significantly different way chemically. For the presumptively rod-connected elements implicated in response to flicker, the curve is markedly discontinuous, so that the high and low temperature parts are dislocated; whereas for the cones they are not. This is entirely consistent with other (e.g., genetic) evidence pointing to their separate physical substrata.
The uncommon exhibition of a higher µ over a higher range of temperature, previously found, however, in a few cases, together with the different relations of rod and cone effects to the critical temperature, explain aspects of these data which in earlier incomplete measurements were found to be puzzling.