The electroretinograms recorded from pure-cone retinae differ markedly from those recorded from pure-rod retinae. Thus, the pure-cone electroretinogram shows a very definite negative a-wave followed by a positive b-wave and a further positive deflexion when the stimulus is removed-the off-effect. There is no c-wave (Chaffee & Sutcliffe, 1930;Bernhard, 1941). The pure-rod or roddominated retina, on the other hand, has little or no sign of an a-wave and a b-wave immediately followed by another slow wave which may be positive or negative (Parry, Tansley & Thomson, 1953). There is often no off-effect, especially if the records are taken during dark-adaptation (Granit, 1933).It so happens that the only pure-cone retinae hitherto investigated belong to cold-blooded species, the horned toad and the tortoise, while most of the pure-, or almost pure-, rod eyes have been taken from mammals.The only mammals known to have a pure-cone retina occur in the family Sciuridae which includes the tree squirrels, ground squirrels and flying squirrels. Of these the flying squirrels are nocturnal and are said to have nearly pure-rod retinae, the ground squirrels are definitely diurnal and are said to have pure-cone retinae, while the tree squirrels are generally diurnal and have a retina which, on-purely histological grounds, probably, but not certainly, contains cones only (Walls, 1942).Since the common grey squirrel, Sciurus carolinensis leucotis, was the easiest of the diurnal Sciurids to obtain, this was the species used, in spite of the slight doubt as to the histological nature of its visual cells. We were interested to examine the general form of the electroretinogram and to determine whether this changes according to the state of adaptation, as is common for the records from mixed rod and cone retinae (Granit, 1933). We were also anxious to know whether the spectral sensitivity curve is that characteristic of rods or of cones and whether it shows a Purkinje shift.
