ACOUSTICS AND GRAVITATION. 



47 



< 



P 



fr 



68 



Finally, the plan of quadratic interferometry is indicated in figure 69, 

 L being a beam of white light from a collimator, with fine slit, *M, M', N, N', 

 the mirrors N and M' half-silvered and identically thick. Small fringes 

 (o.i mm. in the ocular) suffice. The pipe is in position at gpPp'g' and G is a 

 glass compensator. In order that the fringes may be of any desirable size and 

 at any inclination (horizontal preferable), two plate compensators C, C r , 

 revolvable on vertical and horizontal axes, may be installed. If, however, 

 the mirrors M', N' turn conveniently on horizontal axes, and M, N on vertical 

 axes, the former may be used to give the fringes a horizontal trend and the 

 latter, thereafter, for enlarging them, or vice versa. A plane-dot-slot device 

 with spring usually suffices for horizon- 

 tal axes; but the vertical axes require 

 a tangent screw of some form, operating 

 against a return spring. Fringes are 

 normal to the line joining the points of 

 light seen with the naked eye on M' . 

 They are largest when these coincide. 

 Compensators may be moved by hand. 



When the pipe sounds sharply, the 

 achromatic fringes necessarily vanish. 

 Hence they must be observed at T by a 

 vibration telescope (Carnegie Inst. Wash. 

 Pub. No. 249, Part III, Chap. V, and Part IV, Chap. VI, 1919), in which case 

 magnificent wave-forms appear, measurable in amplitude. 



41. Observations with crossed pipes. When the pipe P, figure 67, sounded 

 its fundamental as softly as possible, the even horizontal band of fringes be- 

 came definitely sinuous. At the limit of audition there would be no response, 

 even with much larger fringes. A strong fundamental makes the double ampli- 

 tude about a fringe or more in width. The waves of the overtone are corre- 

 spondingly shorter and high. The adjutages measured I 21 cm. between 

 plates. Reducing this to / = 14 cm., the fundamental came out much stronger, 

 but the loud overtone gave a more confused record. Without adjutages the 

 fundamental (/ = 5 cm.) still evoked very marked waves, but the response of 

 the shrill octave had naturally quite vanished. Moreover, the form of the 

 waves, obtained here without any mechanism, but with the even harmonics 

 deleted, is of additional interest. 



42. Deductions. In an earlier paper (Carnegie Inst. Wash. Pub. 149,?. 145), 

 and apart from details, it was shown that for a length of tube / containing 

 homogeneous air, the density increment Ap for the wave-length X may be 

 written Ap = (C/lR)n\ where C=io 7 Xi.27is the optic constant p / GU O i ) 



IT 



*The fringes at rest are to look like beads on a string, 

 fringes on either side of the middle one, which is white. 



There are about three colored 



