102 , : PRACJ^AJj .PHvOTO-MICKOGRAPHY 



The vertical adjustment of the arc is controlled by screw D 

 and lateral adjustment by the milled head C. The milled head 

 A allows of lateral movement of the positive carbon-holder 

 so that the positive crater is caused to form towards the 

 front of the lamp. It is essential, in selecting any of these 

 hand-feed arc lamps, to see that whatever arrangement is 

 decided on, there is a complete absence of shake, and no 

 unsteadiness of movement or displacement other than in the 

 direction desired, when either of the adjustments are brought 

 into play. 



The particular means by which the carbons are moved in 



relation to one another is 

 not of great importance, 

 provided that it is such 

 that both carbons are 

 moving towards each other 

 at an equal rate, and 

 that no lateral movement 

 occurs. It is, however, 

 essential that there should 



B " r^'SMViL^ -^^R be a means of moving the 



carbons simultaneously, so 

 that without alteration in 

 their relative position, the 

 arc is itself moved as a 

 whole. This, however, is 

 not an easy matter, unless 



there be some means of observing the alteration in the length 

 of the arc or in its position. This can be easily provided 

 for by arranging what is in fact a simple pin-hole camera at 

 the side of the arc, as shown in Fig. 31. A pin-hole at the 

 end of the tube nearest the light throws an image of the two 

 carbon points on to the small ground- glass screen shown at 

 the other end of the tube. Consequently, any alteration in 

 the length of the arc or in the position of the positive 

 crater can be immediately seen by direct observation on 

 the ground-glass screen. It is only necessary to provide 

 on it suitable reference cross -lines to enable the observer 

 to at once adjust the position of his source of light, and 

 to maintain it in an absolutely constant condition and 



FIG. 32. Hand-feed Electric-arc Lamp. 



