506 TRANSACTIONS OF SECTION A. 







7. Photographic Records of a String's Vibrations and Responsive Motions 

 in the Air. By Professor E. H. Barton. D.Sc, F.R.S.E., and 

 J. Pen ZEE. 



When a musical .string is struck, bowed, or pluclsed, the consequent motion oi 

 the string itself is known from the analysis and experimental researches of 

 Ilelmholtz and others. But the quality of the sound perceived by an auditor is 

 not dependent simply on that due to the relative intensities of the partials present 

 in the .string's motion. For the greater part of the sound received does not come 

 directly from the .string, but from .sympathetically vibrating parts of the instru- 

 ment on which the string is mounted. And as the bridge, sound-box, and contained 

 air in turn respond to the motion of the string, probably each introduces its 

 own partialities and exaggerations into the aggregate of vibrations communi- 

 cated to it. It is therefore desirable to ascertain how the motion of each member 

 of this series of moving parts is related to that of the string. The first instalment 

 along these lines ' dealt with the string of a monochord and the belly or top of 

 its sound-box. The present paper is a second instalment of the worl:, and deals 

 with the string of the same instrument and the motion of the air in and out of the 

 central hole of the sound-box. 



To record the motion of the air, this hole was covered with a thin animal 

 membrane, the motion of whose centre pulled and pushed an aluminium stalk, and 

 so rocked a small plane mirror. The axis of the mirror was a sewing needle held 

 in a horizontal position by brass brackets on the vertical face of the sound-box. 

 The mirror reflected the light from an arc lamp to a photographic plate about 

 feet distant. Thus the rocking motion of the mirror caused the spot of light to 

 rise and fall. A simultaneous uniform horizontal motion of the plate along rails 

 drew out the trace into the desired displacement-time curve forming the record oi 

 the vibrations in question. In the positive print this is a bright wavy line on a 

 dark ground. The string's motion was recorded as a black line on a white ground 

 by its crossing the real image of a slit focus.sed first upon the string and then upon 

 the sensitive plate. Thus the two photographic traces are simultaneously obtained, 

 and show the relation of amplitude and phase which held at the time of exposure. 

 On the negatives the magnification of the diaphragm's motion is about 4,200 times, 

 and that of the string's about three times. Thus on the reduced prints, though 

 each is diminished, the relative magnification of the diaphragm to the string 

 remains the same, namely, about 1 ,400. 



8. I/aidinger's Tvfis. By Professor W. F. Bakrett, F.R.S. 



In 1844 the distinguished geologist and physicist Dr. W. Ilaidinger, of Vienna, 

 discovered the curious fact that when a bright sky or any brightly illuminated 

 surface was looked at through a Nicol's prism^a pair of small yellow cones, joined 

 apex to apex, was seen in the direct line of vision. At right angles, and filling 

 the larger space on each side of these cones, a faint blue or violet colour was seen. 

 Ilaidinger called this appearance by the German word biischcl, which means 

 a tuft — in French houppe. Strangely enough, they have been incorrectly termed 

 ' brushes ' in English, and are usually know^n as ' Haidinger's brushes.' " The yellow 

 tufts resemble an hour-glass ; and 1 would suggest the name oi fasckula lutea for 

 them, inasmuch as this would have the advantage of a verbal connection with the 

 macula lutea, or yellow spot, of the eye, with which they are closely associated. 

 For, as is well known, the origin of the yellow tufts does not arise from any cause 

 external to the eye; they are not an e.voptic, but an entojitt'c pheuomenon. How 



' PMl. Mag., July 1905. 



- Sir D. Brewster, in his Optics, p. 246, speaks cf them as ' brushes,' ' tufts,' 

 ' sectors,' and in several places as ' bushels ' ! 



