156 Intelligence and Miscellaneous Articles. 



(c) The best combination of common lenses is two plano-convex 

 lenses, the plane sides of both towards the eye. The constants of 



indistinctness then are — and — — . 

 126 126 



(/) For a single lens to produce the same power, the smallest 



values of constants of indistinctness would be and . 



126 126 



The following rule, though not strictly accurate, will be found 



sufficiently accurate to give a very good practical determination of 



the curvatures of all eyepiece lenses in all cases. Trace the course 



of an excentric pencil through the eyepiece. Consider separately the 



convergence, &c. of the axis of the pencil with regard to the axis of 



the telescope, and that of the rays of the pencil with regard to the 



axis of the pencil. "When both these convergences fall on one side 



(as in the Huyghenian field-glass), the lens ought to be meniscus. 



When they are at equal distances on opposite sides, the lens ought 



to be equiconvex. When the convergence of either is much nearer 



(the other being on the opposite side), the side of the lens next it 



ought to be plane. — Monthly Notices of the Roy. Astronom. Soc. 



Dec. 12, 1862. 



ON THE DURATION OF THE COMBUSTION OF FUSES UNDER DIF- 

 FERENT ATMOSPHERIC PRESSURES. BY M. DUFOUR. 



Hitherto we have had but few data in reference to the influence 

 of the atmospheric pressure on the activity of combustion. More- 

 over, what data we have sometimes appear contradictory ; thus in 

 1841 M. Triger noticed a more rapid combustion of candles in a 

 medium where the air was under a pressure of three atmospheres ; 

 while Prof. Frankland in a recent ascent of Mont Blanc did not per- 

 ceive any essential difference between the combustion of candles at 

 Chamounix and on the top of the mountain. 



In 1855 Quarter-Master Mitchell of the English Navy contributed 

 to the Royal Society experiments made at different heights in the 

 Himalayas with fuses. His results show that the duration of the 

 combustion increases as the pressure decreases ; the combustion 

 appeared less active under a less pressure. Prof. Frankland repeated 

 and confirmed Mr. Mitchell's experiments. He used fuses of six 

 inches from the Woolwich Arsenal ; these fuses were burned in a 

 close vessel in an atmosphere which could be artificially exhausted. 

 In Frankland's experiments the pressure necessarily varied a little 

 between the commencement and the end of the combustion ; and, 

 spite of the ingenious arrangements which he adopted, it was to be 

 feared that the combustion was influenced by the restricted dimen- 

 sions of the space in which it took place. In the month of last July 

 I investigated the duration of the combustion of fuses under con- 

 ditions different to those under which the English physicist worked. 

 I operated in the open air, seeking at different heights ou the Alps 

 gradually lower pressures. 



