THE PRESENT PAPER REPORTS some preliminary experiments on the Ditchburn-Riggs effect which is obtained with stabilized images. Our results are such as to show that the original discovery, made independently by Ditchburn and Riggs and their collaborators about 1952, has opened a new and valuable avenue of approach to the analysis of visual perception.In normal visual fixation, die image that falls on the retina is never really stable; "physiological nystagmus," the continuous tremor of the normal eye at rest, causes a slight but constant variation in the rods and cones that are excited. It is now known that the variation plays a vital role in perception, for it was shown by Ditchburn and Cinsborg (1952) and Riggs, Ratliff, Cornsweet and Comsweet (1953) that stabilizing the image (experimentally eliminating variability of retinal excitation) leads rapidly to the disappearance of the visual object, followed by intermittent reappearanceIn their experiments, the target was projected on a screen after being reflected from a small mirror attached to a contact lens worn by the observer. Thus each slight involuntary movement of the "fixated" eye would produce a movement of the target. By having the subject observe through a complex optical system, it was possible to make the two movements correspond exactly: the angular extent and direction of the eye movement were matched by the movement of the target, cancelling out the normal tremor of the eye and producing a stabilized retinal image. In these conditions the line of demarcation between the two halves of a 1 degree field, separately lighted so as to give intensity ratios of up to 3:1, disappears intermittently for 2 to 3 sec., at intervals of about 1 min. (Ditchburn & Ginsborg, 1952). Similarly, within a few seconds of stabilized viewing, a thin black line crossing a bright 1 degree field fades out; coarser lines are seen for longer periods, but still intermittently, the length of time the line remains visible being a direct function of its thickness (Riggs et al., 1953).

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The subject matter of this chapter does not really form a separate department of psychology. Physiological and animal psychologists are concerned with the same problems and the same theories as other psycho logists. Physiological psychology is distinctive, however, in its use of a particular kind of method and in its conviction that when one has postu lated some process intervening between sensory stimulation -and the consequent behavior (to permit theory to deal with facts that would otherwise be discrepant with it), one has the scientific obligation to sub ject the hypothesis to every feasible test, and to examine the properties of "intervening variables" as directly as possible. Otherwise, error may often be needlessly perpetuated.Animal psychology likewise is not a theoretically distinctive zone, but comparative psychology is distinctive in much the same way that physio logical psychology is; and it is comparative psychology that is discussed in the present chapter. Animal psychology is the psychology of learning, of perception, of emotion, even of thought, and its peculiarity is often only in the use of co-operative subjects that remain conveniently in cages and can be found when they are needed. COMPARATIVE STUDIESThe comparative method, theoretically, is analogous to the physiologi cal method, as 1 correlation of structure with function. Since evolution has provided animals with different structures, neural, sensory, or motor, one may proceed experimentally to discover what features of behavior vary with changes in a particular structure (especially in the central nervous system), just as one may modify structure by surgical operation within a single species and observe the correlated changes in behavior. It has proved difficult in practice to make much use of the comparative method, because one usually cannot separate out one structural variable while controlling others; the effect of size of cerebrum per se, for example, is not known because any great increase in the cerebrum is also accom panied by complex changes in sensory and niotor structures. Few exact studies of this kind have ever been made, but occasional'�ontributions have been of great value. , Another approach, in which the comparison is implicit but still impor tant, involves the study of lower animals in which behavior is (prC9ump tively) simpler. This includes the study of "instinctive" behavior, ann the history of psychology repeatedly shows that higher behavior patterns 1