INTEREST. 698 INTERIM. INTEREST (in ps.vcliology ) • Employed in a very « iilu scn^c, tlu' inlluciice which ofjjcfts exert over the attention. An unusual sound interests us, and nt the same time attracts our attention. In sueli u ease it is not correct to say tlml 'we at- tend bccaimv we are interested,' if l)y this we mean that the interest precedes the attention. In strictness, we are inti'rested in a particular ob- j«-t or line of thoupht only irhilc we are attend- in;; to it. Interest is, then, one aspect of atten- tion. It is the feeling aspect. To he interested is to have ideas which 'feel.' Hence, an object interests us when it alTects us pleasantly or un- pleasantly; though it is more usual to include oniony interesting objects only those which eausB pleasurable feeling. This explains the fact that, althou;;li attention and interest are, in part, iilentical, interest serves to |)r()loiig llic stati' of attention. If an impression alTects us pleasantly, we tend to retain it in mind, it 'holds the at- tention.' Tliis is true to the extent that the greatest voluntary etrort to attend may result in failure, unless an intrinsic interest springs up with the state of attention. The incentives to the arousal of interest are the same as the conditions povernin^ attention ((|.v. ). The objective c<mditions include novelty, intensity, duration, movement, and the 'vital' importance of certain impressions (an animal is fascinated by its enemies and its prey; a man is interested in his kind I. The subjective condi- tions of interest depend upon mental constitution (q.v.). Tile "iMirn mechanician' is always inter- ested in machinery and the professional man is interested in whatever pertains to his profession. Interest is most easily aroused where an object has many relations to past experience ; i.e. where it can be brought into contact with knowledge already acquired. The reason is that the feelin<» which is the essential part of interest is awak- ened thniuKli the pnK'csses of assimilation and throui.rh the mood of familiarity (q.v.). This fact is of importance in education. The child cannot learn things entirely new. but only such things as extend and modify his present stock of ideas and feelings. (See Appkhckptiox.) Never- theless, a correct theory of education goes Ix-ypnd that which is merely attractive as an excitant of the child's interest. It is essential that he learn to attend voluntarily, i.e. that he learn to turn toward and fixate objects which do not at- tract from the outset. Thus the man of science, by virtue of his general 'scientific interest.' be- comes absorbed in tbo.sc minutest details of natu- ral phenomena which to the dilettante are dry and uninteresting. Consult: Stoiit. AnnJjitiral Psychology, vol. i. (London. ISnO) ; Sully, //i/moii Mind, vol. iii. (ib.. ian-2) : Ilr.ner, ]'s]/clioIo(jic (Vienna. 1897) : Baldwin, Uniulhook of Psycholofiy, Frclinp and ^'iH (New York. 1801); Adams, nnhartinn I'syrholofiii A/iplirrl In Education (Boston. 1898). INTERFERENCE (from interfere, from OF. cntrcfcrir, to exchange blows, Fr. inlcrffrer, to interfere, from Lat. inter, between -)- ferire. to strike). In any medium capable of transmitting waves, the effect at any point when (wo (rains of waves are passing simultaneously is the sura of the effects which each wave by itself would pro- duce, provided that the amplitudes of the waves are small compared with the wave-lengths; so that the resultant motion is the combination of the two waves. This is called 'interference.' The interference of two trains of waves on the surface of water is often seen. The ])henonn-non of 'beats' in acoustics (q.v.) is due to inlt-rli-rence. Waves in all media, air, water, the ether, etc., can be made to interfere; and, eonverstdy, if interference phenomena o<cur in connection with any physical action, it is evident that this action must be due to wave -motion. One of the simplest cases of interference is when two iilentical trains of waves are sent out from two vibrating sources close together. At any point whose distances from the two sources differ by half a wavelength, by thni' half wave- lengths, or by any odd numi)er of half wave- lengths, the action of one train of waves is per- manently neutralized by that of the other. Vhereas, at any point whose distances from the two sources are equal, or dilTcr by a whole wave- length, or by any numl«-r of wliolc wave-lenglhs, the action is twice what it would be. due to either train of waves by itself. Thus, let light of one definite wave-length, i.e. of a definite color, e.g. yellow, be passed through a narrow slit in an opaque screen and fall upon a se<'ond opaque screen in which there are two other narrow slits parallel to the first and at equal distances from it; if this pair of slits are close together, and if the light from them falls upon a distant screen of some kind, there will be bands of colored light upon it. separated by ilark bands where llii> ac- tion of the waves from oni' slit is neutralized by that of the waves from the other. The device of having the two slits illuminated by light com- ing from the first slit is in order to secure two identical sources of waves; for if (he sources were to send out different waves, or waves with- out any definite phase-relation, there could be no permanent interference. In a perfectly similar manner waves in the air produced by whistles may be made to give 'interference bands' of silence and sound. The distance apart of the bands in any case evidently varies directly as the wave-length of the trains of waves. See Light. Another case of interference is seen in the beautiful colors of soap-bubbles, of thin films of oil on water, etc. These colors are due to the fact that when white light falls upon the thin trans- parent film, some is rellected at the first surface and of the light that enters the film some is reflected at the second surface, and the rest transmitted. There arc. of course, a series of internal reflections: and looking at the film from cither side the eye receives waves which have passed over paths of different lengths. If (lie difference in phase is such as to amount to half a wavelength or any odd number of half wavi- lengths of waves or any definite wave-length, they will be absent from the light received by the eye. But white light from which any train of waves is absent will appear colored, having the color complementary to that of the absent waves. IN'TERIM (Lat., in the meantime). The name given to three attempts made in Germany during the Reformation to draw up a formula which might serve as a basis of agreement be- tween Catholics and Protestants until (he points of difference could be decided by a general com- eil. The Rntishon Interim was the result of the deliberations of a commission appointed during
