41 



the interior of a glass globe of about one-fourth pint size, which 

 has been very perfectly exhausted of air by means of the effec- 

 tive mercury pump devised by Crookes, as an improvement on 

 the original mercury pump of Sprengel. There were four dif- 

 ferent forms of this incandescent lamp exhibited, bearing the 

 names of Edison, Swan, Maxim, and Lane Fox. Swan's lamps, 

 which were used for lighting the Congress Room, the British 

 Bureau, and the Buffet, behaved exceedingly well all the time, 

 and, as far as I could judge, were absolutely free from any flick- 

 ering. Some of the others met with mishaps which prevented 

 them from appearing to full advantage ; but I believe that all 

 of them when properly supplied with current are equally steady. 

 Their light is yellower than that of an arc lamp, though not so 

 yellow as gas. The little carbon filament in its incandescent 

 condition is so bright that, by what optical writers call irradia- 

 tion, it appears from one-eighth to one-fourth inch in diameter. 

 You can light any number of them, or extinguish them in- 

 stantly ; and arrangements are sometimes provided by which 

 you can turn them up or down to any degree you please by 

 what looks like an ordinary tap. The Swan lamp has been 

 adapted by Mr. Crompton to miners' use, and has been success- 

 fully employed in coal mines. 



The arc lights require two rods of carbon, which must be a 

 small distance apart, and the current passes between their ends, 

 so that not only are the ends of the carbons luminous, but there 

 is also an incandescent space of air, filled probably with little 

 particles of carbon, between. Unlike the filament of carbon of 

 which I have just been speaking, these carbons burn away, and 

 need to be constantly fed forward towards each other, so as to 

 keep them at the proper distance, on which the brightness of 

 the light depends. This " feeding " of the carbons is effected 

 automatically by a great variety of regulators, bearing the 

 names of different inventors, the intention being that when the 

 distance is too great or too small it shall immediately be recti- 

 fied before the difference has produced any noticeable effect on 

 the amount of light. But it is very difficult to do this with 

 sufficient promptness and smoothness, and hence one can almost 



