1890-91.] Sir G. G. Stokes on Suspended Matter in Flames. 263 
On an Optical Proof of the Existence of Suspended 
Matter in Flames. By Sir G. G. Stokes, Bart., E.R.S. 
{In a letter to Professor Tait.) 
(Read June 15, 1891.) 
8 Belgrave Crescent, 
Edinburgh, June 13, 1891. 
Dear Professor Tait, — I write to put on paper an account of 
the observation I mentioned to you to-night, in case you should 
think it worth communicating to the Royal Society of Edinburgh. 
In the course of last summer I was led, in connection with some 
questions about lighthouses, to pass a beam of sunlight, condensed 
by a lens, through the flame of a candle. I noticed that where the 
cone of rays cut the luminous envelope there were two patches of 
light brighter than the general flame, which were evidently due to 
sunlight scattered by matter in the envelope which was in a state of 
suspension. The patches corresponded in area to the intersection 
of the double cone by the envelope, and their thickness was, I may 
say, insensibly small. Within the envelope, as well as outside, 
there was none of this scattering. The patches were made more 
conspicuous by viewing the whole through a cell with an ammoniacal 
solution of a salt of copper, or through a blue glass coloured by 
cohalt. In the former case the light from the flame was more 
weakened than the scattered light, which was richer in rays of high 
refrangibility ; in the latter case the patches were distinguished by a 
difference of colour, the patches being blue, while the flame (with a 
suitable thickness of blue glass) was purplish. The light of the 
patches exhibited the polarisation of light scattered by fine particles 
— that is to say, when viewed in a direction perpendicular to the 
incident light it was polarised in a plane passing through the beam 
and the line of sight. 
When the beam was passed through the blue base of the flame 
there was no scattered light. A luminous gas flame showed the 
patches indicating scattered light like the flame of a candle, but 
less copiously. They were not seen in a Bunsen flame or in the 
flame of alcohol, but were well seen in the luminous flame of ether. 
