254 PROJECTION MICROSCOPE [ CH. IX a 



tion were made with (2) a short circuit would be formed. The 

 absolutely necessary parts in the installation of a lantern are : the 

 main wire conveying the current to the apparatus (i), the main wire 

 conveying the current from the apparatus (2) ; some resistance or a 

 rheostat (3) somewhere in the circuit ; and the ordinary lantern or 

 the projection microscope (5 or 6), (see A of Fig. 204). 



A rheostat of some form is absolutely necessary. This part of 

 the outfit should be adjustable and thus enable one to use a current 

 ranging from 8 to 20 amperes. If a fixed rheostat is used probably 

 one giving about 12 amperes would be most generally satisfactory. 

 It must be remembered that the amount of current at a given moment 

 depends upon the voltage and also upon the amount of current being 

 used by others on the line. For really satisfactory projection one 

 must have an adjustable rheostat. It is also_very desirable to have an 

 ammeter in the circuit, then the current may be regulated with some 

 intelligence, and one can determine whether defects in the illumina- 

 tion are due to insufficient current or to some other cause. If the 

 amount of current is shown to be sufficient by the ammeter, the 

 cause of defective projection will be known to be somewhere in the 

 apparatus and it can be hunted down and corrected. Without an 

 ammeter one is tempted to increase the current for every defect in the 

 projection. One must remember also that the current is only one of 

 the factors in successful projection. 



While a rheostat is absolutely necessary one can omit the am- 

 meter. If it is omitted the main conducting wire (i) would take the 

 course indicated by the dotted line just at the right of the ammeter 

 (4). It should be said that the arrangement of the wires and the 

 rheostat and ammeter in the diagram is a good one, but not the only 

 arrangement. The real point to remember in installing the apparatus 

 is that the current in making the circuit from (i) to (2) must pass 

 through the various pieces of apparatus in turn, and not have it pos- 

 sible for the current to make any short cuts (short circuits). 



403. Using two Lanterns. (Fig. 204 A). If two lanterns 

 are to be used alternately the wiring should be as shown in Fig. 206. 

 For using two lanterns in this way the most convenient switch is the 

 double pole double throw form (7). The two main wires (-f-i and 

 2) are connected with the binding posts at the base of the switch 

 blades. 



Either lamp may be put in the circuit by turning the lever toward 



