Light; a course of experimental optics, chiefly with the lantern . Fig. 24. effect. On now exciting the fork by a violin bow, the spotis expanded by the angular motion of its mirror into a ^ These are the usual arrangements for a gas-burner ; but where thelime-light is used, it will often be preferable so to place this accuratelyin the condenser focus as to make the whole beam parallel, to removethe objective altogether, and place the proper apertures on the front ofthe nozzle, as is usually done with an electric lantern. A rough tin capis easily made in which the zinc plates can slide. Such p
Light; a course of experimental optics, chiefly with the lantern . Fig. 24. effect. On now exciting the fork by a violin bow, the spotis expanded by the angular motion of its mirror into a ^ These are the usual arrangements for a gas-burner ; but where thelime-light is used, it will often be preferable so to place this accuratelyin the condenser focus as to make the whole beam parallel, to removethe objective altogether, and place the proper apertures on the front ofthe nozzle, as is usually done with an electric lantern. A rough tin capis easily made in which the zinc plates can slide. Such parallel beamsare not focused. In all cases the mirror used should be as large aspracticable, as the brightness of the focused spot will depend on thesize of the pencil of light reflected by the mirror. These remarlisapply generally to the class of experiments der^ending upon reflectionfrom a small mirror. D 2 .36 LIGHT. [chap. bright vertical line; and by slightly turning the reflector bin its vertical socket, this is developed, as before, into a. beautiful undulatory form c d, showing each vibration ofthe fork. We may, as is well known, substitute a second II.] THE REFLECTING MIRROR. 37 fork fixed horizontally for the mirror b, and thus get beauti-ful compounded curves; but this belongs rather to anothersubject, and a pair of really accurate forks are beautiful optical effects may be got by squaring oftcarefully the end of a springy steel rod, cementing a smallmirror on that, and fixing the other end firmly by a screwinto a mass of metal sliding on one of the pillars. This ispresented end on to the nozzle of the lantern, in theposition of the fork-mirror a in Fig. 24, but the reflectingmirror b is not moved. On now drawing aside the rod,and releasing it, or striking it, beautiful curves will be pro-duced, of which Fig. 25 may serve as specimens, dependingon the thickness and length of the rod, where it is struck,&c. This is the simplest screen adaptation of S
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Keywords: ., bookcentury1800, bookdecade1880, bookidcu3192403121, bookyear1882
