neighbourhood of strongly Illuminated Bodies. 219 



city at an appreciable distance outside the black coat. It 

 may be of interest here to state that on the surface of ice 

 the coat and plane are absent, and the velocity distribution, 

 considered as above, is apparently uniform up to a point so 

 near the surface of the solid as to be indistinguishable from it 

 with any degree of certainty. 



We must now proceed to consider these appearances in 

 greater detail, and the conditions under which they undergo 

 modification. The contents of this paper may be conveniently 

 summarized under the following heads: — 



1. Experiments made with a view to ascertain the influ- 

 ence of size, shape, and nature of surface of the body exa- 

 mined. 



2. Experiments made with a view to ascertain the effect of 

 temperature, electric potential, and other conditions of the 

 body. 



3. Experiments made on bodies of different material. 



4. Experiments made to observe the effect of pressure on 

 the smoky medium. 



5. Experiments made to examine the behaviour of different 

 kinds of smoke — («) volatile, (/3) non-volatile. 



6. Experiments on bodies which themselves give off smoke 

 or vapour. 



7. Experiments made to examine the conditions under which 

 dust settles on surfaces. 



8. Experiments made in media of different natures, e. g. 

 different gases and liquids. 



1 . Experiments on the Influence of Size, Shape, and 

 Nature of Surface. 



(a) Size. — To see whether the size of the body had any 

 marked effect on the width of the dark coat and plane, we 

 have used cylinders of various thickness (e. g. iron wires *15 and 

 •4 centim. diam., carbon rods # 7 and 1*22 centim.); but without 

 temperature-measurements it is useless to exactly specify the 

 results. We judge, however, that the thickness of the dark 

 coat is independent of the size of the body (other things being 

 the same), but that the rising dark plane is best marked on 

 rods of moderately large diameter ; certainly its base is 

 bigger. 



(b) Shape. — In our early experiments, when examining the 

 appearances of the dark plane, we used copper scraps cu^ and 

 bent into very various shapes; but we foand the dark coat on 

 all of them, and that the up-streaming of the dark plane was a 

 simple matter of stream-lines ; so we need only specify the 

 results for a few of the more important shapes. In the great 



