On the Vibrations of an Atmosphere. 173 



It is apparent from the context that he intends to use the 

 value of N which gives the first bright ring in homogeneous 

 light. - This is the very mistake into which I myself fell. 

 But there is another mistake. Through some oversight the 

 value of N. with which he was supplied by Prof. Stokes, is 

 0'7655, which applies to the first dark ring when the diffracting 

 aperture itself is a narrow ring of diameter d *. When the 

 diffracting aperture is circular or, what comes to the same 

 thing, the diffracting obstacle is spherical, the value of N for 

 the first bright ring is 1*64. It happens that the two errors 

 nearly compensate each other; so that he obtains <i = 0'00162 

 millim. 



If the diffracting particles had been very thin circular 

 disks floating with their planes horizontal, the ring would 

 only have been circular when the sun was in the zenith. 

 When the sun was at an altitude of 30°, the vertical angular 

 diameter of the ring would have been about twice as great 

 as the horizontal. Nothing like this was observed, so we 

 must suppose the particles to have been either nearly spherical 

 or else lying with their planes in all positions at random. 

 A small dilatation of the ring was observed about sunset, 

 especially of the vertical radius, amounting to 3° and 6° at 

 zenith distances of the sun 85° and 90° ; but it is difficult 

 to conceive any shape or orientation of the particles which 

 would explain this. 



As the sun gets low the higher regions of the air become 

 relatively brighter, so that if these contain smaller particles 

 the radius of the ring will increase. On the other hand, as 

 the red rays become relatively more powerful, the red ring 

 will become brighter and broaden out towards the sun. This 

 seems to agree with what was observed. 



Hotel Buol, Davos, 

 November 30, 1889. 



XVII. On the Vibrations of an Atmosphere. 

 By Lord Eayleigh, Sec. U.S.] 



IN order to introduce greater precision into our ideas 

 respecting the behaviour of the Earth's Atmosphere, it 

 seems advisable to solve any problems that may present thorn- 

 selves, even though the search for simplicity may lead us to 

 stray rather far from the actual question. It is proposed here 

 to consider the case of an atmosphere composed of gas which 



* Encyc. Brit., Wave Theory, p. 432. 

 t Communicated by the Author. 



Phil. Mag. S. 5. Vol. 29. No. 177. Feb. 1890. 



