CH. IX] PROJECTION MICROSCOPE 253 



turn on the current and observe which carbon is the more brilliant. As 

 shown in figure 205 the brilliant carbon is at the positive pole. If now 

 the upper carbon is brilliant the wires are properly connected ; but if 

 the lower carbon is the brighter, the wires are inserted in the wrong 

 binding posts and should be reversed. In observing the carbons when 

 the current is on, one should use deeply colored glass to avoid injuring 

 the eyes. Another excellent method is to turn the current off after a 

 minute or two and look at the carbons directly. The one at the pos- 

 itive pole will be red or white hot while the other will be black or very 

 dull red. 



§406. Character of the Carbons and Steadiness of the Light. 

 — One needs a steady light for photography and for projection. To hold 

 the crater in one position and thus render the light steady, a softer 

 core is placed in the carbons (Fig. 204). This serves as a guide, and 

 the crater forms symmetrically around it when the carbons are in a 

 proper position (Fig. 205). Every one who wishes to make a success, 

 of micro-projection is urged to study the appearance of the carbons by 

 using a low objective (35 to 65 mm.). The real image of the carbons 

 formed by the achromatic condenser next the object (§ 399, 4) can be 

 focused and thrown on the screen as if it were an object and one 

 can study the crater. The image on the screen will be right side up 

 as the achromatic condenser inverts it once and the objective rein verts 

 it (no projection ocular being used). 



If the ordinary hard carbons, without soft core, are used the crater 

 shifts its position and thus gives a wavering light. The soft cored 

 carbons give a very steady light as the crater remains constant in 

 position. 



Sometimes a small hard carbon is used for the lower or negative 

 side and a. large soft cored carbon for the upper or positive pole. This 

 works admirably. The sizes used by Ayrton were for the positive car- 

 bon 13 mm. diameter ; for the negative 11 mm. 



§ 407. Rapidity of Wear in the Carbons. — If one employs two 

 carbons equal in size and composition the positive carbon wears away 

 twice as fast as the lower one, therefore one will find the feed 

 mechanism in many lamps moves the upper carbon down twice as 

 fast as the lower carbon moves up. This keeps them centered. 

 If one uses a large carbon above and a small one below, and if 

 the sizes of upper and lower carbon are properly selected, the two car- 

 bons wear away equally in length and the feed mechanism of the lan- 

 tern should move the upper carbon down and the lower one up at the 

 same rate to insure constant centering. 



