254 PROJECTION MICROSCOPE \_CH.IX 



The rapidity of wear in the carbons, irrespective of their position, 

 depends upon the amount of the current. It is uneconomical to use 

 more current than necessary, both on account of the cost of the elec- 

 tricity and the wear of the carbons. It is inconvenient to change the 

 carbons too frequently. It is certainly inconvenient to be compelled 

 to insert new ones during a demonstration. 



§ 408. Size of Carbons and Amount of Current. — The size 

 of the carbons must be proportioned to the amount of current used. 

 For an amperage not exceeding 15, a soft cored carbon of 12 mm.(j4 

 inch) will answer, but if 20 amperes of current are used then the car- 

 bon should be 16 mm. (or f inch) in diameter. If one uses too small 

 a carbon for the current, the carbon partly burns, and really less avail- 

 able light is produced for the projection.* 



§ 409. Amount of Current for Micro-Projection. — For a lec- 

 ture room holding 200 and a screen distance of 8 meters (26 feet), one 

 can demonstrate almost any suitable preparation with a current of 10 

 to 12 amperes, the voltage being no. This serves for objectives as 

 high as 3 mm. when no ocular is used. According to Behrens (Zeit. 

 wiss. Mikr. , 1898, pp. 7-23) one cannot make available more than 20 

 amperes for any micro-projection. The makers of micro-projection 

 apparatus almost invariably make a 20 ampere lamp the limit. 



For getting good results it is vastly more important to have all 

 the parts of the apparatus centered and the carbons in the proper rela- 

 tive position than to use a powerful current. The light cannot serve 

 for projection unless it is properly used (§ 406). 



One will be surprised to see how excellent the results are with an 

 amperage of 10 or 12 when one makes the most of the light. With 

 some preparations one needs more light, and must increase the amper- 

 age. Remember that the skill of the operator is of equal account with 

 the amperage. Do not expect the lantern to furnish brains as well as 

 light. 



§ 410. Lamp Condenser. — This is a large condenser next the 

 radiant and it serves to collect the light emitted from the crater and 



*For the experiments made in preparing this chapter, and for practical use 

 during the last two years, the carbons most employed are designated : "High 

 grade Electra, Nurnberg carbons, soft cored, 12X190 mm. Other forms were 

 also used in the experiments, sometimes the upper carbon was soft cored and the 

 lower one solid. With study and careful experiment one can get good results with 

 a variety of currents and carbons. The beginner is advised, however to, start with 

 the carbons recommended and furnished by the makers of the arc lamp which he 

 is to employ. 





