72 



Lord JRayleigh 



[March 21, 



Fig. 1. 



WEEKLY EVENING MEETING, 

 Friday, March 2 I, 1893. 



Sir Frederick Bramwell, Bart. D.C.L. LL.D. F.B.S. Honorary 

 Secretary and Vice-President, in the Chair. 



The Bight Hon. Lord Bayleigh, M. A. D.C.L. LL.D. F.B.S. M.B I. 



Interference Bands and their Applications. 



(Abstract?) 



The formation of the interference bands, known as Newton's Bings, 

 when two slightly curved glass plates are pressed into contact, was 

 illustrated by an acoustical analogue. A high-pressure flame B (Fig. 1) 

 is sensitive to sounds which reach it in the direction EB, but is 

 insensitive to similar sounds which reach it in the nearly perpen- 

 dicular direction AB. A is a " bird-call," giving a pure sound 

 (inaudible) of wave-length (a) equal to 

 about 1 cm. ; C and D are reflectors of 

 perforated zinc. If C acts alone the flame 

 is visibly excited by the waves reflected 

 from it, though by far the greater part of 

 the energy is transmitted. If D, held par- 

 allel to C, be then brought into action, the 

 result depends upon the interval between 

 the two partial reflectors. The reflected 

 sounds may co-operate, in which case the 

 flame flares vigorously ; or they may inter- 

 fere, so that the flame recovers, and behaves 

 as if no sound at all were falling upon it. 

 The first effect occurs when the reflectors are 

 i close together, or are separated by any 

 multiple of J J 2 . a ; the second when the 

 interval is midway between those of the 

 above-mentioned series, that is, when it coin- 

 cides with an odd multiple of J y/ 2 . A. The 

 factor aJ 2 depends upon the obliquity of the 

 reflection. 



The coloured rings, as usually formed 



between glass plates, lose a good deal of 



their richness by contamination with white 



light reflected from the exterior surfaces. 



The reflection from the hindermost surface 



is easily got rid of by employing an opaque glass, but the reflection 



from the first surface is less easy to deal with. One plan, used in 



the lecture, depends upon the use of slightly wedge-shaped glasses (2°) 



