Arc 



L'UOU C p. 



Journal of Applied Microscopy 



When electricity is used it is almost invariably the incandescent 

 circuit, because it is just as efficient and is less dangerous than the 

 arc circuit. The 52 or 110 volt, direct, incandescent current is the 

 most desirable. On such a current a 90° lamp gives the maximum 

 amount of light. A hand feed lamp is ordinarily to be preferred to 

 the much more expensive self-feed lamp. 



The alternating current is sometimes the only one available and 

 may be used very satisfactorily except for the buzzing sound which it 

 invariably makes. One, however, soon becomes accustomed to this 

 and ceases to mind it. With this current the carbons are best placed 

 180° apart, i. e., in line as shown in Fig. 2 B. They should have a 

 soft core and should have the front surface filed flat ; this brings 

 the craters to the front and increases the amount of light as well as 

 lessens the buzz, since the carbons may be kept closer together. 



With the incandescent current it is necessary to use a resistance 

 box or rheostat. These cost, usually, ten or twelve dollars, but by 

 the use of a high resistance wire such as the la la No. 16, handled 

 by Herman Boker & Co., New York, one can be easily made and 

 will cost less than a dollar. The wire should be coiled, making a 

 helix about an inch in diameter and eighteen inches long. This helix 

 can be arranged between two binding posts so that its length can be 

 varied to meet variations in the current. See Fig. 2 C. 



Method of L ^sing the Lantern. Usually one of the objections to 

 the use of the lantern in the class room is the fact that unless the 

 instructor has some one to run the lantern for him he 

 thinks he must stand behind his audience, which is unde- 

 sirable, both from the teacher's and student's point of 

 view. This objection may be overcome by having a wide 

 angled lens and having the lantern towards the front of 

 the room where it can be easily operated by the speaker, 

 who can at the same time face his audience. This is 

 very easily done where a fair sized room is available, by 

 having the lantern on the speaker's desk and the screen 

 at the side of the room, as shown in Fig. 3. In this way 

 about three-fourths of the seating capacity can be utilized 

 and still have the pictures readily seen by all. The great 

 advantage of this method of using the lantern 

 is apparent, especially when only a 



Lime 

 800 c. p. 



Vapor 



;;(iO c. p. 



Acetylene 



few slides are to be shown during 



250 c. p. the lecture period. In our lecture 



Incandescent 

 12.5 c. p. 



Gas 



Oil 



110 c. p. 100 c. 





Fig. 1. — Diagram to show ttie relative candle power of various lights in common use. 



