December 17, 1920] 



SCIENCE 



587 



introduced at the node N, symmetrically and 

 at right angles to the pipe. The effect of this 

 is to lower the pitch to a degree increasing 

 with the length of p. If p is not too long, one 

 may argue that the resilience of air at the 

 node is decreased, and the period lengthened 

 much in the same way in which an increased 

 capacity operates in an electric circuit. The 

 fundamental pitch was depressed about a 

 fourth. The first overtone however occurred 



Another available pipe is simply an open 

 tube to be excited by resonance. This case is 

 in a measure the most interesting of the 

 three. 



Finally the plan of quadratic interfer- 

 ometry is indicated in Fig. 1, L being a beam 

 of white light from a collimator, with fine 

 slit,2 M, M', N, N', the mirrors, N and M' 

 half silvered and identically thick. Small 

 fringes (1/10 mm. in the ocular) suffice. The 



/ 



c^-l 



-^ 



z 



c/lfN 



<# 



^ 



\ 



.Q 



<^' 



<# 



U 



W 



f4^ 



OrV ^ ?V 



w 





cf 



r-^ — ^ 



^ 



at about a tenth above this and came out very 

 shrilly, probably because it coincided with the 

 octave of the unchanged pipe. Moreover this 

 first overtone is probably the fundamental of 

 the small pipes p, p'. Thus this overtone here 

 also presents two nodes of the same kind, both 

 compressed and both rarified, and the optic 

 effects (ray L) are correspondingly intense. 



The other form of pipe (shown in Fig. 3) 

 has the plane of the embouchure at right 

 angles to the axis of the pipe, which is closed 

 at ff by a knife-edged glass plate. The other 

 end may be open, or closed with glass. The 

 path of the interferometer beam is shown at L. 

 Since the distribution of density is simple 

 harmonic, the details are here quite open to 

 computation. 



pipe is in position at g p Pp' g' and G is a 

 glass compensator. In order that the fringes 

 may be of any desirable size and at any in- 

 clination (horizontal preferable), two plate 

 compensators C, C, revolvable on vertical and 

 horizontal axes, may be installed. If howeverr 

 the mirrors M', W turn conveniently on hori- 

 zontal aves, 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. 



When the pipe sounds sharply, the acro- 

 matic fringes necessarily vanish. Hence they 



2 The fringes at rest look like beads on a black 

 string. There are about three colored fringes on 

 either side of the middle one whieh is white. 



