DISSOCIATION. 302 DISTANCE. forms of l^ldi'lniliirutciis /»j/o;/i7it s uiirciis. GrUtz- iier and Hairiier have shown that the amount of casein preeipitated in milk by the addition of acids depends on the degree of dissociation of the latter, and that the presence of salts modified this relation in accordance with the theory of dissociation. Finally, the researches of Jacques Loeb have shown how important may be the ap- plications of the electrolytic theory to the physi- ology of man and of the lower animals. See Electko-Chemistbv, SoLiTiox; Acids. DISSOLVING VIEWS. Pictures on a screen jjroduced by a mechanical device attached to the magic lantern or slereoptieon which causes the image furnished liy one slide to dissolve, as it were, or blen<i into that of the following slide. Two lanterns are generally employed to secure this etTeet. which is obtained by gradually cover- ing one of the projection lenses and allowing the rays from the other to fall on the screen. DISSONANCE (Lat. dissonant ia, from dis- soiiare, to sound harshly, from dis-, away + sonnrc, to sound, Skt. span, to resound). The simultaneous soimding of two or more tones for- eign to a major or minor triail. Dissonance pro- duces a feeling of unrest reiiuiring a resolution into consonance (q.v.). This feeling of unrest is attributed to the complex ratios existing between the number of vibrations of dissonant intervals: whereas the ratios of consonant intervals are al- ways simple. In reality it is impossible to set up any exact limits of dissonance, as what may seem dissonant to one person may seem consonant to another. But it is easy to establish certain landmarks within which each individual will, to a certain extent, detennine the bounds of disso- nance. Dissonant intervals are the major and minor second and all augmented or diminished intervals. (See Interv.vl.) Chords containing one or more dissonant intervals are dissonant chords and require resolution. The dissonant chords are all augmented and diminished triads, as well as all chords of the seventh and ninth. Sec Acoustics; Chord: H.rmoxv: Triad. DISTAFF ( AS. diilaf. from " disc. OE. dizcn, to dress with tlax for spinning, to bedizen, I^w Ger. diesse. flax on the distaff + sl(ef. stafT). The staff on which the flax or wool is fastened, and from which the thread is drawn in spinning. As represented in ancient art. and still used by Greek peasants, it is a straight rod. fnmi either side of which projects near one end a semicircle of cane or withes, forming a bed to which the wool or flax is attached. In both ancient and modern art. the Fates are usually represented with it, engaged in spinning the thread of life. It has ever been considered as the peculiar embleni of feminine as opposed to masculine occupations, and is sometimes used figuratively for a woman. DISTAFF'S DAY, S.mnt. name given to .Tamiary 7. biiaii-e it marks the return of the women to their usual daily occupation after the Christmas festivities terminating on the Twelfth Day. .I:inu;vry fi. DISTANCE (OF., Fr. distance, from Lat. distiinlia. from dislare. to lie distant, from dis-. apart -+- stare, Gk. iirraimi, Iiislanai, OChurch Slav, stati, Skt. sthii. to stand: connected with OHG. sifn. Ger. slrhcn, to stand, and ultimately with Goth., AS. standnn. Engl, stand). In navi- gation (q.v.K the distance lietween any two places is the length of the rhumb-line (q.v.) be- tween the places. On the ordinary sailing chart ( -Mercator's projection) the rhumb-line is straight. Also, the angular distance l)elween heavenly bodies nieasurcd on the arc of the great circle jiassing through them. The limit of view in a jiicturc, or iioint of dis- tance, as it is called in |)crspeclive, is that por- tion of the picture w here the visual rays meet ; the middle distance being the central portion be- tween the extreme distance and the foreground. Distance, in a picture, is obtained by jiainting the tones that express the relative remoteness of a distant object with the same directness and truth that is employed in portraying those ob- jects that are near at band, or in the foreground, as it is called. When distant objects are thus true in color, the quality of distance will be pro- duced by the diminution in size which obedience to the laws of perspective exacts. See Perspect- ive. DISTANCE, or DEPTH, I'ekckptio.x ok. ^^'hen we look at objects we think that we see how far away they are. Yet their distance is not directly given with the act of vision. It is true that a few writers, e.g. Hcring and James, be- lieve that the primitive held of view was not alto- gether Hat. and that distance is 'a genuinely opti- cal felling'; but the majority agree that it is de- rivative, built up by the association of tactual ideas with certain "signs,' visual and strain sen- sations arising in the use of the eyes. In either ease it is admitted that the 'optical signs' them- selves, the 'criteria of distance,' as they are called, are essential to the complete development of the iilca. We must distinguish between the ■primary,' immediately given, and the 'secon- dary,' or indirect criteria. The fonner are (1) dissimilarity of retinal images: (2) converg- ence stnrins; (3) accommodation strains: and (4) dispersion circles. The last must be taken together with the third, .lone. it is of minor importance; for accommodation, which clarifies moderately distant objects, will occasion disper- sion circles for both nearer anil farther objects. The strain set up in the ciliary muscle as it focuses the lens may. however, sen-e as a definite empirical criterion of distance. The second fac- tor results from the functioning of the eyes in binocular vision. If we are to have a single clear image of near objects, the eyes must converge. The intensity of the str.ain sensations thus en- gendered increases in proportion to the proximity of the fixated point. Experimental investiga- tions of the .sensitivity to strain sensations, as well .as the anatomical arrangement of the twelve eye-muscles, warrant the assumption that such sensations mav give accurate indications of the distance of objects, at least up to a few hundred feel. Convergence, together with accommoda- tion, is, according to Wundt and Arrer, the es- sential basis of the percept inn of depth. But other authorities, notably Wheatstone. Tiering, and Tlillebrand, lay stress upon the dissimilarity of retinal images. We have given us. in our two eyes, two pictures of every object seen. The difTernice between the two pictures increases with the nearness of the object. The testing of the part played by this disparity in the perception of distance led to the invention of the stereoscope, which enables us artificially to reconstruct the actual retinal conditions of binocular vision.
