SOUKCES OF ILLUMINATION 99 



too large, the tip of the negative carbon will become blunt and 

 rounded (Fig. 30 b) ; and if too small, a mushroom-shaped 

 excrescence forms on its tip, and the lamp burns unsteadily 

 (Fig. 30 c). In the former case the resistance in series with the 

 lamp requires increasing, so that the current is decreased in 

 amount, while in the latter case the resistance should be 

 decreased, a corresponding increase of current resulting. The 

 distance between the carbons must also be modified either 

 lengthened or shortened to conform to the altered conditions. 



The distance between the two carbon points is of primary 

 importance, and on the careful regulation of this distance and 

 its constant maintenance depends the constancy of the source 

 of light. In the case of a continuous current, the positive pole 

 is the light-producer ; the negative pole only produces half as 

 much light as the other, and, as already explained, this should 

 not be utilised. The positive carbon is also consumed at a rate 

 that is approximately double the rate of consumption of the 

 negative. To obviate the difficulty due to this difference of con- 

 sumption, the sectional area of the positive carbon is made twice 

 that of the negative one, so that the reduction of their lengths 

 is practically constant. In the case of an alternating current, 

 which is for many reasons practically useless for microscopic 

 work, the rate of consumption of the two carbons is equal. In 

 this case too, an additional difficulty is that the two carbon poles 

 are equally incandescent and, consequently, only one-half of the 

 light produced can be used. Further, any method of automatic 

 regulation is much less certain than with continuous currents, 

 and even in hand-feed lamps, the arc is very liable to acquire 

 a periodic rotation round the edges of the craters, which is 

 fatal to its use for photo-micrography. 



Of all electrical illuminants the arc light has the highest 

 efficiency, and it has the advantage that its luminous point is 

 quite regular, so that the image of the light- source in the object- 

 plane of the microscope can be easily obtained, giving an 

 evenly illuminated field. A convenient current consumption 

 is from seven to ten amperes if a light of moderate intensity 

 only is required, but by increasing the current consumption 

 to twenty-five or thirty amperes, a light-source of sufficient 

 intensity for the highest power work is obtained. It should be 

 understood that increase of current consumption does not 



