106 PEACTICAL PHOTO-MICBOGBAPHY 



electrical instruments are by no means necessary, although 

 at times they are a convenience ; but if all the necessary 

 arrangements are made by a reliable and competent elec- 

 trician, as they most unquestionably should be, there will be 

 little need for their use. 



A convenient form of hand-feed arc lamp for use with 

 small currents is that recently introduced by Messrs. Leitz of 

 Wetzlar (Fig. 34). In it the positive and negative carbons 

 are at right angles to one another ; and, although this 

 arrangement cannot be entirely justified on theoretical 

 grounds, it must be admitted that in practice, with a lamp 



FIG. 34. Hand-feed Electric-arc Lamp for a Current of 4 amperes. (E. Leitz.) 



using small carbons, it works extremely well. The tendency 

 with this type of lamp is for the arc to form in a position which 

 does not allow of the maximum light being projected forward ; 

 a great amount is then emitted towards the back, and so is, 

 for practical purposes, lost. It may be that on very extreme 

 tests as to efficiency this small lamp of Messrs. Leitz would 

 not be better than one in which the carbons are approxi- 

 mately vertical, but in actual practical use there is little or 

 nothing to be urged against it. The difficulty usually found 

 in the type with carbons at right angles, due to the 

 lower negative carbon coming in the way of and obstructing 

 the light emitted by the positive carbon, is partially over- 

 come by using a positive carbon which is somewhat smaller 



