As the electric arc is much too bright to be looked into with the naked eye, both concentrating apparatus are supplied with a lens, called a focus or flame observer, by means of which an image of the burning carbons is thrown upon small screens at the back, so that the lamp can be easily adjusted without fatigue to the eye. The focus observer is shown on the lamp in holophote, Fig. 172. _Precautions._--Before starting the apparatus, the electric lamp terminals and those of the dynamo machine must be _connected up_ by means of the leading wires provided with each set of apparatus. The terminals are marked _C_ and _Z_ respectively, and they should be connected, _C_ of machine to _C_ of the lamp, and _Z_ of the machine to _Z_ of the lamp, in order that the electric current may be sent in the proper direction through the carbons of the lamp. Should it, however, be found that the top carbon (which should consume twice as fast as that of the bottom one) does not consume so fast as the bottom one, it may be assumed that the dynamo machine has reversed its poles, and the leading wires will consequently require changing across. This reversal of poles, though possible, is of _very rare_ occurrence. [Illustration: Fig. 173.] The dynamo-electric machine should not be driven without its proper leading wires to lamp and lamp being connected up, or at least an external resistance equivalent to that of the lamp (which is approximately one Siemens' unit) must be inserted. In other words, the machine must not be driven when a wire of small resistance connects the two terminals _C_ and _Z_. This is expressed more briefly by saying the machine must not be _short-circuited_. If it is short-circuited when in motion the electric current becomes so powerful that it will leap from segment to segment of the commutator, where very bright and large sparks will be seen, and if continued would destroy the insulation, thus weakening the current generated. The leading wires should never be disconnected suddenly while the machine is revolving at its full speed, as such a sudden interruption will produce an intense spark, which will burn the ends of the wire where the contact is suddenly broken. When it becomes necessary to disconnect the wires, the belt should be pushed on to the loose pulley by means of the striking gear, or the steam engine should be stopped. It may be here stated that all connections should be cleaned bright and screwed tightly, to ensure perfect metallic contacts being made. _Coupling two Machines._--At Fig. 174 is shown a diagram of how to make the connections when coupling two machines in parallel circuit. _MM'_, _m_, _m'_, represent the ends of the wires of the electro-magnets; _BB'_ are the branches; _C_ and _Z_ are the terminals of each machine respectively. [Illustration: Fig. 174.]