PSYCHOLOGICAL APPARATUS. 508 PSYCHOLOGICAL APPARATUS. wliioli it can be heard: or the distaiue is kept constant and the lieight of fall varied until a sound can just be sensed. Our discrimination of sound intensities is measured by the sound pendulum, or the fall phonometer. The principle of both instruments is the same— two sounds, of slightly different intensities, are produced by tlie droppinfr of a ball or pendulum-bob against a liard-wood block, and the observer is required to say at what point the diirerence becomes notice- able. The highest audible tone is determined either by means of a series of tiny tuning forks, or steelcylinders. or by means of Oalton's tchis- tle, a piston whistle of" small bore actuated by a rubber bulb, whose length can be varied from that required for a shrill tone to a length that gives a sharp hissing noise, with no trace of tonal quality. The lowest audible tone is deter- mineil by a very large tuning fork, giving tones between the limits lli and io vibrations in the second ; by tuning forks of wire, loaded at the tips of the prongs; by a steel tongue. Appunn's lamella, which can be set in vibration lietween the limits 4 and 24 in the second: or by the pro- duction of deep difference tones. I See AiniTlO.v.) Discrimination of tonal pitch and the phenomena of clang-tint and tonal fusion (see FrsiON) are studied by the aid of delicately graduated tuning forks; reeds, with their appropriate bel- lows tables: organ pipes; blown bottles; siren; sonometer; etc. For the study of rhythm, the ticks of a metronome are employed, or the jnilTs of tone from a tuning fork placed behind a rotat- ing disk of cardboard, pierced at regular inter- vals. For the study of localization of sound, i.e. of the apparent distance and direction of a given source of sound, the sound cage is used. This consists of two graduated semicircles of wire, the one turning about a horizontal and the other about a vertical axis. The observer sits with his head at the centre of the semicircles, and attempts to localize the click of a telephone re- ceiver, which is placed by the experimenter at some point upon the circumference of the sound sphere. PsYCiioLOGiCiiL Optics. Psychologv' borrows from the oculists their various tests of the acuity of vision, and from _the physicists their mani- fold photoinetrical devices. The phenomena of color mixture are studied by means of the spec- tropholomrlrr. or (more usually) by the rotated disks of the color-mixer. This consists essen- tially of an axle, vertical or horizontal, which i^ turned with extreme rapidity by hand, by clock- work, or by an electric motor. Disks of card- board or paper, slit along one radius so that they may be fitted together to form a sinale 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 be employed for experi- ments on our sensible discrimination of bright- nesses (grays) and colors. The phenomena of indirect vision are brought out by the perimeter or eampimeter. The former consists of a hollow hemisphere, upon the centre of which the eye to be examined is fixated. Wafers of various colors are l)rought into the hemispherical field, along the different meridians, and the observer reports what he thus (indirectly) sees. When the whole field has been explored, a map can be 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 colorblindness the simplest is, perhaps, the worsted test of Holm- gren. Some hundred skeins of ditl'erently 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 pscudoscopc 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 rersa. 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 arc 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 ; anil yet others for the test and control of the visual imagination. Haptics a>d Organic Sensation. The first re- quirement in the field of cutaneous sensation is a set of pressure, temperature, and pain points. The.se 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 purjiose is the kinesimeter of Hall and Donaldson, which passes pressure or 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
- irea and bending weight of which are known.
Discrimination ifor pressure is measured by weights, laid upon the resting skin; discrimina- tion for lifted weights ( in which jiressure cn- iiperates with the articular and tendinous M-ii>a- 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 algo- 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 cpsthesiomcter. 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 board 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 kin.Testhetic sensa- tions is tested by the finger-movement and the arm-movement apparatus of Cattell and Munster- berg. In both of these the finger is inserted in a car. which travels for a prescribed distance along a track ; the observer then tries to repro- duce the movement by memory. The static sense (sensation of dizziness) is studied by the rotOr
