CH.IXa\ PROJECTION MICROSCOPE 255 



that lamp until connection is made. This arrangement enables one 

 to use either lamp at will. 



A fuse block just before the switch is very desirable. The fuses 

 should carry from 20 to 25 amperes of current. 



404. Arrangement 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-projection 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 have 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 satis- 

 factory light. 



Within the last few years lamps have been constructed to hold the 

 carbons at right angles, the horizontal carbon serving for the illumi- 

 nation. Its brilliant crater faces the lamp condenser, and it is believed 

 that the illumination with this arrangement is more satisfactory than 

 with inclined carbons. 



However the carbons are arranged in the lamp success is attained 

 only when the crater at the positive pole faces the condenser and is 

 on its principal axis, and the negative carbon does not get in the way 

 and thus form a shadow. The proper appearance is shown in Fig. 

 205 E of the 90 degree carbons, and in Fig. 207 of the more common 

 inclined carbons. One can get the right arrangement of the carbons 

 only by experience. The lamp should be adjustable so that the posi- 

 tion of the whole lamp may be changed vertically or horizontally to 

 ensure perfect centering, and then one must be able to change the 

 relative position of the carbons. 



New carbons are very blunt and in many cases need readjusting 

 after they have become sharpened by wear. Their correct position 

 and amount of separation for a 12 to 15 ampere current is shown in 

 Fig. 204. 



The best method for determining the character of the crater is to 

 project the image of the two carbons at the arc on the screen with a 

 low objective (50 to 75 mm. equiv. focus). One can then mutually 



