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236 MICRO-PROJECTION WITH DIRECT CURRENT [CH. 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 condenser 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 projection; 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