64 Proceedings of the Royal Society 
halves of this Sphygmosphone might be removed to any distance, so 
that if connected with a telegraphic wire, the rhythm of the pulse 
of an individual in London might he made audible in Edinburgh. 
Monday, 21 st January 1867. 
Dr CHRISTISON, Vice-President, in the Chair. 
The following Commnnications were read : — 
1. On the Colours of the Soap-Bubble. By Sir David 
Brewster, K.H., F.R.S. 
The colours of the soap-bubble have been the subject of frequent 
observation since the time of Boyle, Hook, and Newton, and they 
have been invariably ascribed “ not to any colour in the medium 
itself in which they are formed, or on whose surfaces they appear, 
but solely to its greater or less thickness.” The author of this 
paper had been led to doubt the correctness of this opinion, and 
while repeating the beautiful experiments of Professor Plateau 
“ On the equilibrium of liquid films,” he w T as led to discover the 
true cause of these colours, whether they are observed on the soap- 
bubble or on plane, convex, and concave films stretched across the 
mouths of closed or open vessels. 
The paper, which is illustrated with numerous coloured draw- 
ings, is divided into five parts. 
1. On the phenomena of colour in a vertical plane film. 
2. On the production of revolving systems of coloured rings on 
the soap film. 
3. On the form and movements of the bands and rings on convex 
and concave films. 
4. On the phenomena produced by different solutions. 
5. On the origin and development of the colours on the soap- 
bubble. 
In these sections the author has shown that the colouring matter 
of the soap-bubble is secreted from the soap solution when reduced 
to the state of a film ; — that it rises to the highest point of the film 
in colourless portions, in the form of a tadpble, which pass into 
molecules of every possible order of colour, and then take their 
