SOUKCES OF ILLUMINATION 107 



than the negative. To compensate for the more rapid con- 

 sumption of the positive carbon, which would of course occur 

 on a continuous-current circuit, the positive feed is more rapid 

 than the negative. As the positive carbon is smaller than 

 the negative and is kept as small as possible for the current 

 used, the size of the luminous crater is, in relation to the 

 total sectional area of the carbon, fairly large ; the crater, in 

 fact, occupies nearly the whole of the end of the carbon itself. 



The lamp is provided with a condensing-lens arranged so 

 that it projects approximately a parallel beam. It also has 

 an adjustment for altering its inclination, so that for visual 

 work, and particularly for use with the modern dark-ground 

 illuminators, it may be inclined at any suitable angle. The 

 feeding arrangements for the carbons are simple and certain in 

 action, and the feed is sufficiently exact to ensure that the 

 carbons shall move in perfect alignment. The lamp is designed 

 for a current of four amperes, but it will work with a consider- 

 ably greater or smaller current consumption than this. It must 

 of course be used with a suitable resistance in series to somewhat 

 suit the particular circuit on which it is intended for use. 



The difficulty inherent in all illuminants, particularly 

 electrical ones of small area and irregular shape, is so to arrange 

 them that the image of the source of light will cover the object 

 under observation. With either carbon or metallic filament 

 electric lamps, or even with the Nernst electric lamp, the 

 filament is of considerable length, but is very narrow ; the 

 consequence is that if a direct image of the source of light is 

 projected it is impossible to fill even a high-power objective 

 with light. A common way of overcoming this is either to 

 enclose the source of light in a ground-glass globe, or to interpose 

 between it and the sub-stage condenser a piece of ground-glass. 

 The objection to this method is that too often the irregular 

 surface of the ground-glass is itself projected into the field of 

 view, and produces an appearance which for visual work is 

 disconcerting, and for photographic purposes is absolutely 

 impossible. Mr. J. W. Gordon has suggested the application 

 of a speculum which overcomes the difficulty and enables a 

 light-source of small area or irregular shape to be used. It 

 consists essentially of a piece of glass rod, one end of which is 

 polished and the other cut to a plain surface and finely ground ; 



