TRANSACTIONS OF THE SECTIONS. 15 
rigin; and as there are various strong arguments in favour of the 
jon that the diamond is a vegetable substance, the new structure 
ch he had described might be considered as an additional argument 
a fayour of that opinion. He had, in a former paper, placed it beyond 
a doubt, that the diamond must have been in a soft state, like amber 
or gum, and capable of having its structure modified by the expansive 
force of air, or gaseous bodies imprisoned in its cavities ; and therefore 
the fact of its being sometimes composed of strata of different degrees 
of induration and refractive power, was more likely to have been pro- 
duced by pressures varying during the formation of the crystal, than by 
any change in the intensity of the forces of aggregation of its molecules. 
Such a change might have been supposed probable in the diamond 
had it been previously found in any other crystal. He had already 
referred to the action which diamond exerts superficially upon light. 
‘Professors Airy and Maccullagh have found that this action is of a 
yery peculiar kind, having some analogy with that of metallic surfaces ; 
but it was obvious, from the preceding facts, that a surface of various 
‘ refractive powers must disturb, in a very considerable degree, the 
phznomena produced by its superficial action. In studying, indeed, 
this class of phenomena, it would be necessary not only to obtain a 
‘surface of uniform structure, but to make the experiments before that 
‘surface had experienced any change from the action of the atmo- 
‘sphere. In surfaces of glass such changes often take place in a few 
days; and the thin films of oxide which are thus created are so thin 
that they can only be rendered visible by examining the light re- _ 
flected from the surface, when it is placed in contact with an oi] or 
liquid of the same refractive power. 
Account of a singular optical Phenomenon, sometimes seen at sunset. 
By Proressor Curistie, Seeretary of the Royal Society. 
_ Mr. Christie drew the attention of the section to an optical phzno- 
‘menon which he had observed at sunset, when looking from the Down 
below the Needles Lighthouse, in the isle of Wight, across the Solent, 
towards the Hampshire coast, and which he had described in a letter to 
Professor Forbes, referred to in the published Reports of the Associa- 
tion. He stated, that he had observed the same phenomenon on sub- 
sequent occasions. The appearance was that of a very distinct vertical 
ray of yellow light, having the sun for its base, of the same diameter 
throughout, gradually diminishing in brilliancy, but very distinctly to 
_be traced to the height of more than 30°. This appearance continued 
for half-an-hour after the sun had set. On two other occasions he had 
observed what he considered to be the same phenomenon. In one 
_ ease he happened to be on Westminster Bridge, and on the other on a 
hill about a mile to the north of the town of Bedford. On both these 
occasions the sun was considerably above the horizon, perhaps 6° or 
7°, the strata of cloud in its vicinity were much denser than on the 
former ones, and the phenomenon did not present the same marked 
