. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. on both the lecturer and theaudience. The heating effects of the current may be shown by carry-ing a coil of platinum wire through water and boiling it;or by making a pieceof platinum wire in-candescent as last method,however, and theeffects of heat in in-creasing resistance,are not as a rule pro-jection experiments,and therefore need not be further referred to. Incandescence as shown in electric lamps is best projectedby Mr. Hopkins method, of showing in the field of thelantern th
. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. on both the lecturer and theaudience. The heating effects of the current may be shown by carry-ing a coil of platinum wire through water and boiling it;or by making a pieceof platinum wire in-candescent as last method,however, and theeffects of heat in in-creasing resistance,are not as a rule pro-jection experiments,and therefore need not be further referred to. Incandescence as shown in electric lamps is best projectedby Mr. Hopkins method, of showing in the field of thelantern the apparatus itself by as dim a light as will make itjust visible on thescreen, and thensending the cur-rent through,when the lightitself will be alsoin focus. is a simplelittle arrangementfor an incandes-cent lamp, andfig. 232 gives anequally simple at-tachment for projecting the carbon points and arc. No regu-lator at all is needed for the latter, as anywhere within aninch or two will project equally well by this method; and therebeing no mechanism to be worked, a smaller current will give. Fig. 232 4io OPTICAL PROJECTION good results. Lamps can, however, only be thus projected bya lantern whose light can be readily and considerably lowered ;for while a dim projection of the non-luminous parts very muchadds to the effect, any brightness in that part of the imageutterly spoils it. If the carbon-points themselves are toobrilliant, a small diaphragm maybe placed over the objective,and the light in the lantern increased accordingly. Anotheradvantage of this method is that the poles are projected sharplyand uncoloured, owing to the employment of the achromaticlantern objective. Ordinarily the arc (in lantern) is projectedby the condenser alone, passing the rays through an aperturesmall enough to tone down too great brilliance. With regard to the mechanical or motor effects of thecurrent, their elements have been already mentioned in § adequate range of current be at
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Keywords: ., bookauthorwrightle, bookcentury1900, bookdecade1900, bookyear1906
