169 
PHYSICAL AND MATHEMATICAL SECTION. 
October 15th, 1863. 
Joseph Baxendell, F.R.A.S., President of the Section, 
in the Chair. 
Mr. Baxen dell, referring to Mr. Brothers’ “Iris Diaphragm” 
for determining star magnitudes, stated that, theoretically, 
the aperture of the diaphragm should vary as the cosine of 
half the angle of rotation of the moveable ring, and exhibited a 
diagram showing the close agreement between the theoretical 
and actual apertures in an instrument he had had constructed 
under Mr. Brothers’ direction, and adapted to Mr. Worthing- 
ton’s equatorially mounted achromatic of five inches aperture. 
He also stated that the only difficulty he had experienced in 
using the diaphragm was that of not being able to read off 
readily the divisions of the scale without artificial illumina- 
tion ; but this difficulty he had found could be obviated by 
the use of a small telescope magnifying about six times, 
which showed the divisions of the scale on the darkest night 
with sufficient distinctness for all practical purposes, and 
thus enabled the observer to make a series of observations 
without exposing the eye to extraneous light or leaving the 
eye end of the telescope. 
Mr. Baxendell also read the following extract of a letter 
from Mr. Heelis, F.R.A.S., dated at sea, off Genoa, Sep. 9th, 
1863 : — 
I promised to write to you if I had anything to com- 
municate. I have very little, but that little relates to a 
rare phenomenon in the Mediterranean, no less than a 
genuine fog. In the last voyage of the Palestine, on the 7th 
