110 British Association for the Advancement of Science. " 
common mode, to within half an inch of the top; then worked out, | 
in the usual way, all air bubbles as perfectly as possible ; filled : 
up the tube to the top and inverted it ina cup of hot mercury, | 
when it, of course, subsided in the upper part of the tube to the 
barometric height ; he then placed his finger on the mouth of the 
tube under the mercury in the cup, and lifted it out; and still 
“holding his finger tightly over the mouth of the tube, laid it flat 
on a table, when the mercury in the tube soon lay at the under 
side of the tube, leaving void the upper part along the lengthof = 
the tube. On turning the table slowly round, still keeping the : 
finger on its mouth, every particle of air was gathered up. He 
then placed the tube upright, with its mouth upwards, and pla - 
_cing a funnel of clean dry paper about the upper part, an assis- 
tant filled the funnel slowly with hot mercury, so as to cover the 
fingers. On slowly withdrawing the finger, the mercury went 
gently in, and almost perfectly the atmospheric air 
which had gathered into the void space. By renewing the pro- 
cess which succeeded the previous washing of the air out of the | 
_ tube, once or twice, a column of the utmost brilliancy was ob- 
ined. Dr. Robinson suggested the substitution of a piece of 
caoutchouc for the finger in this process, and it was found a de- 
cided advantage. ‘The method of procuring an invariable sur- 
face in the cistern was equally simple. He proposed to divide 
the cistern into two compartments, by a diaphragm of sheet iron 
or glass brought to a sharp edge at top. Into one of these com- 
_ partments the tube dips ; in the other-is placed a plunger of glass a) 
or cast iron, which can be raised or lowered by a slow screw JB 
movement. To prepare for observation, the plunger is first’ | 
- screwed down, by which it displaces the mercury in one com- | 
partment, and raises its surface in the other above the edge of | 
the diaphragm ; on raising it slowly again, the mercury drains off 
to the level of the edge of the diaphragm; thus at every obsel- | 
vation, reducing the surface to a fixed level. | 
The following letter was communicated from Sir John Her- 
schel, containing a most interesting communication respecting the 
action of the dissevered rays of light in the solar spectrum. 
- 
ee 
“i e 
My Dear oe Siem I take the liberty of requesting that yos | 
