236 MICRO-PROJECTION WITH DIRECT CURRENT [Cn. IX 



The screen must be as reflecting as possible. Nothing has ever 

 yet exceeded in satisfactory quality a smooth, dull, white, wall. 

 For a full discussion of screens see Ch. XII, 621. 



MICRO-PROJECTION WITH THE DIRECT CURRENT ARC LAMP AS THE 



LIGHT SOURCE 



361. Arc lamp and wiring for the same. The direct current 

 arc light is the only fully satisfactory artificial light known at 

 present for micro-projection. Hence it will be taken as the 

 standard, as with the magic lantern (Ch. I). Furthermore, as the 

 upper carbon is always made positive and hence is the source of 

 light, this carbon is made horizontal and the crater faces the con- 

 denser and is in the optic axis. That is, for micro-projection 

 we take the right-angled arc lamp as the standard (fig. 3, 121). 



The wiring, rheostat and ammeter are as with the direct current 

 magic lantern radiant, (figs. 2, 3, 133). The rheostat should be an 

 adjustable one. The ammeter can be omitted, but it is more 

 important than with the magic lantern, for the conditions of 

 micro-projection must be made as nearly perfect as possible. With 

 the ammeter one can tell instantly whether the proper amount of 

 current is flowing. If there is sufficient current the light should be 

 satisfactory, or if it is not satisfactory it will be due to some fault 

 in optical adjustment. The ammeter is urged upon all users of the 

 projection microscope because the tendency is to run in more and 

 more current if the projection is unsatisfactory, hoping by pure 

 brute strength, so to speak, to overcome difficulties due to improper 

 adjustment. In case one cannot afford an ammeter, then the next 

 best thing is, when installing the apparatus, to measure the current 

 flowing through the arc with the different settings of the adjustable 

 rheostat, and to mark these values on the rheostat dial. One can 

 then set the rheostat at the proper amperage for the given projec- 

 tion; but as the voltage on the line is subject to variation, one 

 cannot be sure that the proper current is flowing at any given 

 moment unless an ammeter is present to indicate the amount. 

 With many lighting circuits, the fluctuations in voltage are very 

 small, and one can be reasonably sure of getting the current indi- 



