. The book of photography; practical, theoretical and applied. om arrangement generally requires twoto four secondary cells in series with theprimary of the indiiction coil. Thestrength of the current in the tube, andtherefore its consequent heating effect(varying approximately with the squaie ofthe currents magnitude), may be regu-lated by tightening the adjusting screw- belonging to the spring of the contactbreaker, or by altering the current in theprimary circuit. A Wimshurst inductionmachine can be used instead of the coil;this will necessitate a motor to keep it incontinuous ro
. The book of photography; practical, theoretical and applied. om arrangement generally requires twoto four secondary cells in series with theprimary of the indiiction coil. Thestrength of the current in the tube, andtherefore its consequent heating effect(varying approximately with the squaie ofthe currents magnitude), may be regu-lated by tightening the adjusting screw- belonging to the spring of the contactbreaker, or by altering the current in theprimary circuit. A Wimshurst inductionmachine can be used instead of the coil;this will necessitate a motor to keep it incontinuous rotation. It should have aLeyden jar connected to each of its primeconductors, and the terminals of theGeissler tube should be attached to theouter coatings of the jars by silk-coveredwires as shown by Fig. 766. These wires,as also those of the secondary circuit ofthe induction coil, should be kept wellapart in air, and the stand holding thegas tube should be thoroughly getting near the wires while thecurrent is flowing, or a disagreeable shock. Fig. 766.—Wimshurst Machine Sparking throughTdbe of Gas. may be received. Do not place the Geiss-ler tubes too near the collimator whenusing large coils, as the spectrometer isgenerally made of metal throughout, anda shock might be experienced on ap-proaching the eye to the instrument. Direct Sunlight. ^ The suns rays are reflected on to theslit by means of a heliostat (or siderostat,as it is often termed). But, even if thelight is required for a fairly long interval,it is permissible to adjust the mirror occa-sionally by hand, to eliminate the effectof the suns apparent motion across thesky, since all that is required is that sun-light should enter the slit. The mirrormust be concave, or a convex lens must beused with a plane mirror to focus the 570 THE BOOK OF PHOTOGRAPHY. rays, so that the amount of light enteringthe collimator is increased. If the lightbe too intense, the lens may be movedso as to give a larger ill-d
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