SIR DAVID BREWSTER 69 



to the Royal Society of Edinburgh on February 18, 1867, Sir William 

 Thomson refers as follows to the genesis of the conception : 



"A magnificent display of smoke-rings, which he recently had the pleasure of 

 witnessing in Professor Tail's lecture-room, diminished by one the number of assump- 

 tions required to explain the properties of matter, on the hypothesis that all bodies are 

 composed of vortex atoms in a perfect homogeneous liquid. Two smoke-rings were 

 frequently seen to- bound obliquely from one another, shaking violently from the effects 

 of the shock.... The elasticity of each smoke-ring seemed no further from perfection 

 than might be expected in a solid india-rubber ring of the same shape.... 



" Professor Tait's plan of exhibiting smoke-rings is as follows : A large rectangular 

 box open at one side, has a circular hole of six or eight inches diameter cut in the 

 opposite side.... The open side of the box is closed by a stout towel or piece of 

 cloth, or by a sheet of India-rubber stretched across it. A blow on this flexible side 

 causes a circular vortex to shoot out from the hole on the other side. The vortex 

 rings thus generated are visible if the box is filled with smoke." 



Then follows a description of one way of producing a cloud of sal- 

 ammoniac, not the way however as generally practised by Tait ; and the paper 

 ends with a description of the effects of collision between vortex rings 

 produced from two boxes. This seems to be the earliest printed account of 

 Tait's experiments on vortex rings which gave the start to Thomson's famous 

 theory of vortex atoms. 



From 1859 till his death in 1868 Sir David Brewster was Principal of 

 Edinburgh University. In spite of his eighty winters the famous experi- 

 menter still continued his researches, and Tom Lindsay, then a youth training 

 as mechanical assistant under his father, James Lindsay, tells how Brewster 

 made considerable use of the optical facilities of the Natural Philosophy Class 

 Room, and discussed many optical phenomena with the young Professor. Sir 

 David had made his residence at Allerly near Melrose and travelled to and 

 from Edinburgh by train whenever his University or Royal Society duties 

 demanded his presence. Had he lived in Edinburgh, he would no doubt have 

 spent a large part of his time in the Natural Philosophy Department; for Tait, 

 then as ever, cordially welcomed any one who had a physical problem to 

 investigate. Among the subjects which specially occupied Brewster's attention 

 during the later years of his life were the colours of soap films and the pheno- 

 menon which he had discovered in 1814 and had described under the name of 

 the Radiant Spectrum. When a bright small image of the sun, such as may be 

 obtained by reflexion from a convex mirror, is viewed through a prism, there 

 appears in addition to the usual spectrum a bright radiant spot beyond the 



