PSYCHOLOGICAL OPTICS. Psychology borrows from the oculists their various tests of the acuity of vision, and from _the physicists their mani fold photometrical devices. The phenomena of color mixture are studied by means of the 8/ac t/Top/two/peter. or (more usually) by the rotated disks of the color-mixer. This consists essen tially of an axle, vertical or horizontal, which is tented with extreme rapidity by hand. by clock work, o• by an electric motor. Disks of card hoard o• paper, slit along one radius so that they may be fitted together to form a single compound disk, are clamped upon the near end of the axle, and the colors of the disks cancel or blend, according to their quality and saturation. Color-mixers may also he employed for experi• meats on our sensible diserimination of bright nesses (grays) and colors. The phenomena of indirect vision are brought out by the perimeter or cam ',buffer. The former consists of a hollow hemisphere. upon the centre of which the eve to be examined is fixated. Wafers of various colors are brought into the hemispherical along the different meridians. and the observer reports what lie thus (indirectly) sees. When the whole field has been explored, a map can he made of the three zones of retinal sensitivity. The eampimeter does the same work as the per imeter. except that the field is plane and not hemispherical. Of tests of color-blindness the simplest is, perhaps. the wo•.sted teat of Holm gren. Some hundred skeins of differently colored wools are brought together, and the subject is required to match. from the heap. certain skeins that are handed him by the experimenter. For the study of perception of space in the third dimension, the stereoscope and pseudoscope are indispensable. The latter is a converting stereo scope, i.e. a stereoscope whose lenses or prisms give us an illusion of inverted tridimensionality, hollows appearing in relief, and rice rerun. This department of psychological inquiry is rich in research instruments, which expose threads, lines, or edges at different (and, of course, unknown) distances from the observing eye. There are also special instruments for the sudden brief expos ure, on a dark field, of words or figures, which are to serve as the starting-point of a train of association; others for the serial exposure of words or colors, which are to be memorized; and yet others for the test and control of the visual imagination.
HArnes AND ORGANIC SENSATIoN. The first re quirement in the field of cutaneous sensation is a set of pressure, temperature, and pain points. These are of wood, metal, or hair. Various means have been devised for regulating the in tensity of pressure. the temperature of the ap
plied point, etc. One instrument for this purpose is the kinesimeter of Hall and Donaldson. which passes pressure o• temperature points over a selected area of the skin, at a constant rate and intensity. The least noticeable pressure is deter mined by a series of small and accurately graded touch weights, or by a series of hair points. the area and !mending weight. of which are kinnvn. Discrimination for pressure is measured by weights. laid upon the resting skin; discrimina tion for lifted weights (in which pressure co iiperates with the articular and tendinous sensa tions), by cylindrical weights of hard rubber. filled with shot, which are lifted successively in pairs, and thus compared. Sensitivity to tem perature and pain is determined by the applica tion of temperature cylinders and of the alga meter to a chosen portion of the skin. The algo meter is a rod, usually covered at the exposed end with cloth or chamois leather, which works against a spiral spring; the amount of pressure which evokes pain is read from a scale laid along the spring. The msthesiometer, in simplest form, is a pair of ordinary drawing compasses, tipped at the ends with hard rubber. The object of the instrument is to show us how far apart two cutaneous pressures must lie, if we are to per ceive them as separate; and. again, what separa tion of the compass points at one part of the skin gives a separation. in perception, equal to that of a given separation of the points at an other part of the skin. The least amount of movement that can arouse an articular sensation is given by the arm board. a hinged hoard upon which the observer lays his hand and arm (bring ing the elbow over the hinge). and which is then. very gradually. raised or lowered by the experi menter. Discrimination of kinaesthetic sensa tions is tested by the finger-movement and the arm-movement apparatus of Cattell and lliinster berg. In both of these the finger is inserted in a ear. whieh travels for a prescribed distance along a track: the observer then tries to repro (Nee the movement by memory. The static sense (sensation of dizziness) is studied by the rota Lion table, a fiat bed or table upon which the observer is stretched. and which can be twirled round, in the horizontal plane, at constant rates. Perception of the movement and position of the whole body is investigated both by the rotation table, and by the tilt-board, a similar instrument, in which the table can he swung through approxi mately ISO' in the vertical plane. The observer, strapped upon the table, estimates the extent and direction of movement, the true values of which can be read from a scale.