PSYCHOLOGICAL APPARATUS. The ap paratus employed in a well-equipped psychological laboratory falls into the following main divis ions: (1) Physiological models of the sense or gans and the brain; (2) demonstration appara tus, for use in the lecture room before a large audience; (3) drill apparatus, for class work with students in the laboratory; and (1) re search apparatus for the investigation of new problems, generally built, in part at least, within the laboratory. To these four classes may be added (5) certain anthropometrical instruments ( see ANTHROPOMETRY), such aas those which measure the diameter of the pupil, or the force and steadiness of muscular action; ((I) appa ratus for the observation of the habits and faculties of the lower animals, such as a micro scope, with special attachments for work on the protozoa; mazes of wire or wood, to test the formation of habits in reptiles or small mam mals; aquaria: cages, whose fastenings are de signed to test the intelligence of their occupants; and (7) simple instruments for use with chil dren or defective persons, designed to test sensa tion, perception, feeling, action, attention.
Psychological instruments proper, i.e. the pieces included in classes (3) and (4), may he divided again into two great groups, as qualita tive and quantitative. The object of a quali tative instrument is to demonstrate a fact. Sup pose, e.g. that we have a black box, with ground glass front, containing a gas-lamp, and that there are two sets of grooves behind the glass, the front set for the' insertion of a black wooden shutter, and the rear set for the insertion of a second glass, colored. We light the lamp, pull up the shutter, and let the observer stare at the colored glass, which he sees through the ground glass front. After thirty seconds we drop the shutter, and the observer secs, on the dark gray of the ground glass, a 'negative after-image' of the original color. If the colored glass was green, he now sees a colored pa tell of deep pur ple. We have performed a qualitative experi ment upon the vjsual after-image; we have demonstrated its existence, but we have not measured its duration, or its intensity and extent as compared with the intensity and extent of the green stimulus. Had our apparatus been so
constructed that we could take these measure ments, the experiment would have been quanti tative.
The instruments designed for quantitative work are of two kinds: those which furnish a direct scale-reading, and those which show the course of a bodily or mental process, with all its variations, as a function of time elapsed. In struments of the former type are familiar to every one. The mercury thermometer, e.g. is a physical instrument, which allows one to deter mine the temperature of a room by noting the point on the scale of degrees that has been reached by the head of the column of mercury. Instances from psychology would he the a'sthesi ometer, which tells us how many millimeters apart two points must be set down upon the skin, with a given intensity, if they are to be perceived separately- as two, and not run together in a single blurred perception; the chronoseope, from the dials of which we can read MI our re action time in units of a thousandth of a second: and the protractor of the color wheel or color mixer, which tells us the number of degrees or half-degrees by which we have varied the com position of a parti-colored rotating disk. in all these cases we have a scale of conventional units, from which we can read our results. Instru ments of the second type are those employed by the 'graphic method.' The essential feature of this method is that it furnishes a curve the ab scissa- of which are time units (seconds, or fifths o• tenths of seconds), while the varying heights of the' ordinates show the variations of the process under observation. The process may be bodily, as when we trace the curve of breathing. in order to see how it changes with change in our mental state: o• it may he mental, as when we have recourse to the graphic method to record the fluctuations of attention (q.v.).