104 PEACTICAL PHOTO-MICKOGKAPHY 



the crater occupies a large portion of the end of the carbon. 

 The tendency is for the arc to take the shortest course across 

 the gap between the two carbons, and consequently it may 

 happen that more light is projected behind than in the direc- 

 tion of the microscope. It has the decided advantage that, 

 after the necessary preliminary adjustment, the positive crater 

 always remains central, since the consumption of the carbons 

 takes place in the direction of the optical axis. 



It must be clearly recognised that any form of automatic 

 regulation, unless it is of the very best construction, is worse 

 than useless. Evidently in the hand-feed arrangement there 

 is no mechanism to get out of order, but in the automatic 

 the whole efficiency of the apparatus depends on its at once 

 responding to small changes in the current flowing through the 

 lamp. 



Of necessity an automatic lamp is intermittent in action, 

 and its mechanism is only brought into play as the result of 

 some lengthening of the arc and consequent alteration in the 

 amount of current ; how much change is required to set the 

 carbons in motion depends entirely on its sensitiveness. It is 

 therefore difficult to ensure the constancy and accuracy of move- 

 ment which can be obtained by a simple hand-feed apparatus. 



An automatic lamp generally takes some ten or fifteen 

 minutes for the carbons to burn themselves into shape, and 

 even that time may be exceeded if the current that is being 

 used is greater or less than the lamp is designed for. 



It will be advisable to give some brief instructions as to the 

 method of connecting up an arc lamp to the source of electrical 

 supply. Unless there is already extra heavy wiring sufficiently 

 large to carry the current, it will be absolutely essential to 

 have special wires brought, preferably from the distributing 

 switch-board, to the point where the lamp is to be run. These 

 wires will require to be considerably thicker than those used for 

 ordinary lighting purposes, and should have fuses, or what are 

 known technically as ' cut-outs,' exactly suited to the current 

 proposed to be used. 



Fig. 33 indicates the method of connecting up the lamp. 

 The wires are brought from the terminals on the board shown 

 at A, respectively positive and negative, as marked. .They 

 then pass successively to the safety fuses, and from thence to 



