582 REPORT— 1894. 



Depaetments II., III. 



A joint meeting with Section I. was held to discuss the two following Papers 

 hy Professor Oliver Lodge : — 



10. On Experiments illustrating Clerk Maxwell's Theory of Light. 

 By Professor Oliver Lodge, F.R.S. 



11. On an Electrical Theory of Vision. 

 By Professor Oliver Lodge, F.R.S. 



Depaetment II. 



12. On the Velocity of the Cathode Rays. 

 By Professor J. J. Thomson, F.R.S. 



13. On a Ten-canclle Lamp for use in Photometry. 

 By A. Vernon Harcourt, M.A., F.R.S. 



The author has in former years brought before this Section a burner consuming 

 a mixture of air with pentane vapour, and lamps consuming pentane vapour, which 

 gave a constant amount of light equal to that of an average standard candle. 



When so small a light is used in the ordinary photometry of coal-gas, errors 

 from defects in the adjustment of the photometer, or from slight haziness in the 

 atmosphere of the testing-place, are greater than when a light is used more nearly 

 of the same magnitude as that of the gas-flame which is measured. A light of 

 ten candles is well suited for the purpose. It has been shown that such a light 

 can be obtained from an argand gas-burner consuming air saturated with the 

 vapour of pentane ; and a gas-burner has been proposed for use which is well 

 suited for this purpose, except perhaps in being of a rather complex structure. 

 But to supply air saturated with pentane vapour at a steady pressure there is 

 needed a gas-holder of a few cubic feet capacity. The addition of such a gas-holder 

 to the apparatus makes it more costly and not easily portable. It has been shown 

 that great variations may occur in the proportions of pentane and air consumed by 

 the gas-burner without materially aSecting the light given out by the lower part 

 of the gas-flame. The admixture of air is, however, unnecessary, since at a 

 moderate temperature pentane can be volatilised without there being any necessity 

 for reducing the atmosplieric pressure by the admixture of another gas. The lamp 

 shown is on the same principle as the one-candle lamp devised by the author seven 

 years ago. The wick is raised or depressed by the ordinary rack-and-pinion 

 movement, the lower end of it dipping into pentane in the body of the lamp, while 

 the upper end is warmed by the heat conducted down from the flame. In the ten- 

 candle burner the wick fiUs the circidar interspace between the central and outer 

 tubes of an argand. The only difference between this and an ordinary lamp is 

 that the wick does not come near the flame, and needs no cutting or renewal ; nor 

 does the smoothness or roughness of its upper surface afl^ect the burning of the lamp. 



The dimensions of the lamp and chimney upon which the air currents inside 

 and outside the flame depend have been determined by experiment so as to produce 

 a bright and steady flame. The entrance to the inner tube is throusrh a triangular 

 opening as in the usual construction of such lamps. It was found that the free 

 admission of air at this entrance caused an inequality in the flame, two peaks 

 appearing on either side of the place of admission of air. To steady and regulate 

 the admission of air at this point, a cylindrical case is fixed round the lower part 

 of the tube, into which air is admitted through a round hole 15 mm. in diameter. 



The flame of a rich gas burning from a wide opening is of the colour of candle- 

 light even when an abundant supply of air is provided on both sides of the flame. 

 The light of a good gas-burner is less red, or more blue, than this ; and it is 



