and Laboratory Methods. 193^ 



or 90°. If an alternating current is used, a more or less pronounced humming 

 sound will be heard, but lecturer and audience soon become accustomed to it. If 

 the direct current is used, care must be taken to so connect the cables leading 

 from the switch to the lamp that the horizontal carbon will be positive. An easy 

 method for determining the positive carbon is to allow the lamp to burn for a 

 moment or two, then open the switch and observe the carbons. One will be 

 seen to be more luminous because more highly heated and it is the positive car- 

 bon. If it is also the horizontal carbon the connections are correctly made ; if 

 not, detach at the lamp and change the ends of the cables to the other carbon. 

 It is well, at first, to exercise special care lest one put his body in a circuit by 

 touching the two carbons, or their holders, or other uninsulated parts at the same 

 instant. A 110 volt current running through one's body is not likely to produce 

 serious results, if indeed it has any effect other than to convince one that it is 

 not a desirable mental stimulus for one who is lecturing. Currents from arc 

 systems are not as desirable as from incandescent systems on account of the 

 danger attending their use. 



When using a direct current, the horizontal carbon, or the one which fur- 

 nishes the available light, must be positive. To test it, open the switch, after 

 the lamp has been lighted about a minute, and look at the points of the carbons. 

 One carbon will be seen to be more heated and so remain red longer than the 

 other. The hotter carbon is positive and, if it is not the horizontal one, reverse 

 the wires at the lamp. With alternating current the carbons are equally 

 luminous. 



Having tested the lamp and obtained a good light, proceed as follows to 

 adjust the lamp and microscope for projecting. Fill the water tank with clean 

 water from which the air has been expelled by boiling for a few minutes and 

 then cooled in an air tight fruit can. Leave a small air space in the top of the 

 tank to allow for expansion as the water becomes heated during use. See that 

 the condensers are clean and set squarely in their places and that all of the opti- 

 cal parts are in the same optical axis. Set the lamp so that the point of the 

 vertical carbon will be three inches from the adjacent face of condenser number 

 one, and adjust the tip of the horizontal carbon as accurately as possible in the 

 optical axis of the lenses. Put on a low power objective, e. g., a }i-'m. or 1-in., 

 remove the sub-stage condenser and amplifier, set the microscope carrier at such 

 a position that an object on the stage of the microscope will be about twelve 

 inches from the vertical carbon, measured along the optical axis. These 

 measurements are approximate and intended simply as a guide in making the 

 first test of the instrument. Place a mounted object, or a slide on which an ink 

 line has been drawn with a pen, on the stage and turn on the current at the 

 switch. Strike the arc, as directed above. As the available light is derived 

 from the horizontal carbon, care must be taken that the vertical carbon is not 

 too high so as to intercept the light. Focus the objective carefully and proceed 

 as follows to test the light for its maximum efficiency. If the circular field on 

 the screen is not evenly illuminated, raise or lower the entire lamp or move the 

 arc to the right or left until the illumination is central. Slide the microscope 

 carrier slowly toward or away from the light and clamp it at the position giving 



