title: Signals from cones.
abstract: 1. We have studied red and green cones by contrast flash inhibition and found them both to be very similar to rods in their response to flash energy, except that all light quantities must be some 100-fold greater in cones for the same effect.2. Using the methods of a previous paper (A.R.T. a) where no backgrounds were employed, we plotted log test flash lambda against log surround flash varphi with criterion that lambda should just be detected. The experiment was repeated with a ;windmill stop' interposed in the varphi flash which reduced its area symmetrically to (1/8). From these two curves it is possible to extract the relation between N, the inhibitory signal, and varphi, the test flash. It isN = varphi/(varphi+sigma),where sigma, the semi-saturation constant, is about 4.5 log td sec.3. Using the methods of (A.R.T. b) where backgrounds were studied, we measured the increment threshold for the surround flash varphi against its background theta using as criterion not that varphi should just be seen but that it should generate a fixed inhibitory signal N so that the fixed test flash lambda could just be seen.4. This increment threshold curve resembled the Aguilar & Stiles (1954) curve for rods, showed saturation and a complete symmetry about the 45 degrees line through the point with co-ordinates (log theta(D), log sigma).5. These results imply that the cone signal N is related to flash varphi and steady background theta by [Formula: see text], where theta(D) is receptor noise (= eigengrau), and varphi and theta are expressed in units of quantum catch.6. The ordinary increment threshold for cones does not show saturation because a steady saturating background bleaches all the pigment away. When the background is presented for only 100 msec with dark pauses between, no great bleaching occurs and saturation is seen.