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 in a 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 length of 

 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 fuimel 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 displaced 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- 

 tained. 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 

 or cast iron, which can be raised or lowered by a slow screw 

 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 obser- 

 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. 



My Dear Sir, — May I take the liberty of requesting that you 

 will mention to the Physical Section of the British Association a 

 very remarkable property of the extreme red rays of the Prismatic 



