164 OPTICAL PROJECTION 



called the Brockie-Pell, but of much simpler construction ; 

 and the same lamp can be adjusted from about 7 to 15, or 

 from 10 to 20 amperes of current, according to its size. I 

 have seen it work for two hours without a single ' blink.' 



The effect of the inclined carbons is shown in fig. 88. 

 The lower carbon is still set in front of the upper or positive 

 carbon, according to the length of arc. This brings the 

 crater ' considerably to the front of the carbon, while the 

 Incandescent portion of the negative carbon is behind, and 

 invisible from the front. Hence the radiation to the condensers 

 is reduced, as shown in fig. 88, to that from the crater, while 



that crater radiates much 

 more directly and powerfully ; 

 the general result being, not 

 only the one radiant point, 

 but that of so much greater 

 power, that a lamp which 

 would usually be called of 

 2,000 candles' power, gives to 

 the condensers at least 3,000 

 in proportion. The gain in 



FIG. 88 11 -, , - , 



all microscopic and high-class 

 optical work cannot be over-rated. 



The management of this lamp is very simple. The upper 

 carbon, first set by experience the proper distance behind the 

 other, is pushed down by hand till it just touches the lower 

 one without pushing that down ; as the current when switched 

 on must draw it down a small distance in the usual manner by 

 an electro-magnet, and so strike the arc. The position as to 

 height of the point of the lower carbon, and therefore of the 

 arc, can be adjusted within considerable limits. The current 

 will first have been ascertained, at least approximately. If 

 the current is sufficient to work the lamp to its full power, 

 the milled head which regulates the arc, seen behind the 

 upper carbon pillar on the top of the lamp case, is turned to 



