CH. IX] PROJECTION MICROSCOPE 251 



when the current is established to separate them slightly (1 to 2 mm. ) 

 in order to get a satisfactory light. The carbons may be brought in con- 

 tact before turning on the current or afterwards. An automatic lamp 

 will start as soon as the current is turned on, but here also the carbons 

 must be slightly separated, or one must wait a short time for the car- 

 bons to wear away before the best light is obtained. 



§ 402. Angle of the Carbons. — Originally arc lamps for use 

 with the lantern had the carbons both vertical. The "projector" lamps 

 used at sea, had, however, the carbons inclined at an angle of 30 or 40 

 degrees from the vertical. Lewis Wright (Optical Projection, p. 163,) 

 states that at his urgent request a projection arc lamp for micro-projec- 

 tion was made with inclined carbons. Certain it is that all projection 

 lamps have now the inclined carbons. The angle of inclination varies 

 with different makers. The lamps furnished by Zeiss and Reichert 

 with their apparatus has the carbons at 40 degrees from the vertical ; 

 Behrens uses and recommends 45 degrees. Barnard and Carver (J. R. 

 M. S., 1898, p. 170) found by a careful series of experiments that an 

 angle of about 27 degrees gave the most satisfactory light. One firm 

 ( A.T. Thompson & Co. of Boston) arranges the carbons at right angles, 

 the upper or positive carbon being horizontal. This gives good results. 

 The purpose of the inclination is to turn the crater toward the con- 

 denser ( Fig. 205), for it is now appreciated that the arc proper gives 

 very little light comparatively. One will appreciate this by studying 

 the carbons projected on the screen as suggested in § 406. 



§ 403. Adjusting the Carbons. — In many arc lamps for projec- 

 tion there is a metal shelf to show approximately where the two car- 

 bons should meet. If one places the carbons in their holders so that 

 the ends are at the level of this shelf they will be nearly in the correct 

 position. 



The lower carbon should be slightly in advance of the upper one 

 (Fig. 204). This will insure the formation of the crater on the side 

 facing the condenser. 



S 404. Length of the Arc. — It has been found by careful inves- 

 tigation (Carhart, Ayrton) that the maximum brilliancy and efficiency 

 of a continuous current arc lamp are obtained when the arc is about 1 

 mm. long and the current is about at its maximum for the size of car- 

 bon used. If the carbons are too far apart the light becomes purplish. 

 If the current is too weak the lower carbon is blunt, while with a 

 stronger current it becomes more pointed, and hides less light ; it also 

 contributes a share of the illumination from its white tip (Fig. 205). 



