TRANSACTIONS OF THE SECTIONS. 3. 
broad at the top and shallow. When the summits come within the limb, then, by 
the same causes, they will be melted down, or at least be reduced to their natural 
proportions. But the encroachment of the solar light on the dark disc will be 
greater towards the sides, where it is at a greater depth from the edge of the sun, 
and there will be thus a general protuberance towards the point of contact. 
As to the cause of the diminution of the sun’s light at the edge we are in entire 
ignorance. Again, dark glasses by diminishing the light may destroy the effect of 
irradiation, and the power and aperture of the telescope are known greatly to in- 
fluence its amount. Hence it is quite conceivable that under different conditions 
the phenomena may be greatly modified, or may not appear at all, 
—_——- 
On a new Case of Interference of Light. By the Rev. Prof. Powerit, F.R.S. 
The principal experiment evincing this new kind of interference consists in placing 
a plate of glass or other transparent substance in a prismatic vessel containing a 
fluid (as e.g. oil of sassafras or anise with plate or crown-glass), so as to intercept 
the upper or thicker half of the prism, when the spectrum is seen covered with dark 
bands parallel to the edge of the prism, the number and breadth of which vary 
greatly with the refractive powers of the plate and medium, and with , 
the thickness of the plate. For many combinations the plate must Fig. 1. 
be inserted in the way just described, or towards one end of the 
spectrum, thus exhibiting an effect analogous to what was termed 
“ polarity ’’ in the experiments by partial interception of Sir D. 
Brewster: as in fig. 1. But for many combinations no bands are 
produced by this arrangement. In these cases however, on placing 
the plate to intercept the thinner part of the prism, as in fig. 2, 
bands will be produced. 
This remarkable relation, as well as the number and character of 
the bands, can be all expressed by a formula derived from the simple 
interference theory; but for some more minute changes observed, 
recourse must be had to the diffraction theory, as in Mr. Airy’s in- 
vestigations (Phil. Trans. 1840, 1841). These investigations have 
been pursued by Mr. Stokes, of Pembroke College, Cambridge. 
When plates of doubly refracting crystal are employed, two sets 
of bands are seen superimposed, even in those of the most feeble 
doubly refracting power, as quartz, &c. This may perhaps be ser- 
viceable to the mineralogist for detecting this property when very - 
weak. 
In general the number of bands observed in different cases agrees sufficiently well 
with calculation, and the method may be applied inversely for finding the refractive 
indices of one substance, the other being known. There is also a close analogy 
between these bands and those described by the Baron von Wrede, though pro- 
duced in a totally different manner. [See Taylor’s Scient. Mem. vol. i. pt. 3. 
p: 487.] 
Observations of the Annular Eclipse of October 9, 1847. 
Collected by the Rev. Prof. Powri1, F.R.S. 
The British Association having at the last meeting printed and circulated “‘ Sug- 
gestions for the Observation of the Annular Eclipse,’ a copy of which is also in- 
serted in the last volume of its Reports (Sectional proceedings, p. 16), it has ap- 
peared desirable that at the present meeting a short statement should be laid before 
the members of the observations made, as far as intelligence has been received. - 
Unfortunately the morning was cloudy over nearly the whole of that part of Eng- 
land in which the eclipse was annular, so that numerous observers who might have 
been expected to respond to the invitation of the Association, had no results to 
communicate. On some parts of the east coast of Kent the eclipse was partially 
seen, but not (as far as has been ascertained) in the annular phase. 
It being uncertain whether Greenwich would or would not fall within the limit 
of annularity, the Astronomer Royal, with the aid of several scientific friends, 
B2 
