
“wht nia 
_ pupil or of the object-glass. 
TRANSACTIONS OF THE SECTIONS. 25 
ternal fringes are represented by mn, m!n', and are convex outwards, or parallel to the 
sides of the point MN. The inéernal fringes, 
as seen by Grimaldi and Dr. Young, are shown 
by the deep black lines between A and B. 
These internal fringes, however, I have ob- 
served extending far beyond the shadow in 
fine hyperbolic curves, as shown between o 
and p, and o and p’. They intersect the ex- 
ternal ones, and give them the appearance 
of screws or twisted cords. In homogeneous 
light, where the fringes are alternately dark 
and coloured, the dark fringes are dark at 
their intersections, and the coloured ones coloured. 
When the needle-point is illuminated by the spectrum, and the fringes viewed by 
a lens, which is necessary to see them, we require to approach the lens to the fringes 
m'n! on the violet side of the spectrum, and to withdraw it on the red side, in order 
to see them distinctly. When this experiment is made with great care, I have 
counted fwenty external fringes on each side of the shadow, which may always be 
seen most distinctly by looking through the margin of the lens. 
When the diffracting: body is an exceedingly small wire with parallel sides, the 
internal fringes extend far beyond the shadow, mingling with the external ones, and 
completely altering their colours and forms. 
The internal fringes beyond the shadow, like those in it, disappear by intercepting 
the light with a screen on the opposite side of the diffracting body. 
In former notices on certain dark bands in the spectrum, the author noticed that 
they resembled screws or twisted lines; and he infers from the preceding experi- 
ments, that these bands may have a similar origin, that is, that they may be pro- 
duced by the intersection of two systems of bands, or by portions of bands produced 
by the ragged or uneven edge of the diffracting body. 
In this communication Sir David stated that the bands which in former notices he 
had considered as indicating an apparent polarity of light, were merely the internal 
diffraction fringes produced by the edge of the plate, displaced, according to M.Arago’s 
discovery, by the retarding action of the plate itself, and rendered visible through 
the action of the prism in one position of the edge more than another. He had seen 
the fringes under various circumstances, whether the diffracting edge was towards the 
red or blue side of the spectrum, never having any dependence on the aperture of the 



On four simultaneous Experiments in the Island of Bombay to determine the 
Fall of Rain at different Hei ghts below 200 feet. By Dr.G. Burst. Com- 
municated by Col. SYKEs. 
‘Dr. Buist gave the details of the means which he adopted to ensure accurate re- 
sults. No satisfactory conclusion could be drawn, because the gauges at the several 
heights below and at 200 feet did not give uniform results; sometimes the most 
elevated gauges having the greatest fall of rain, and at other times the lower 
gauges having the greatest quantity. Nor did gauges at similar heights receive the 
same quantity of rain. 
On Atmospheric Daily and Yearly Fluctuations. By Dr. G. Butst. 
The object of the author was to show from the annual and daily oscillations of 
the barometer south of latitude 44° N., that the usually received opinion, that ‘‘ the 
annual range of the barometer increased and that the daily fluctuations decreased as 
the equator was receded from,”” was met by so many instances to the contrary as 
to go far to invalidate the rule. Dr. Buist supplies a list of twenty-five stations 
from lat. 43° 39’ N. to lat. 42° 52’S., in which the annual and daily ranges of the 
barometer are given for the year, and for the summer and winter months. With 
the exception, however, of Aden, Toronto, St. Helena, and Hobart Town, the stations 
are confined to the continent of India and within 22 degrees of latitude north of the 
