Light; a course of experimental optics, chiefly with the lantern . tern up perpen-dicularly through the circularaperture. Owing to the largersize of this (the horizontal field)a diverging beam will generallybe required, from any ordinarylantern, to cover it; and hencethere should be a large plano-convex lens (costing about one 6 inches in diameter)fixed to the under side of theaperture, so as to re-convergethe rays on the focusing effects can, however, beprocured without this with agood lime-light, in spite of whatis then wasted. In any case,all but the condensers are re-move
Light; a course of experimental optics, chiefly with the lantern . tern up perpen-dicularly through the circularaperture. Owing to the largersize of this (the horizontal field)a diverging beam will generallybe required, from any ordinarylantern, to cover it; and hencethere should be a large plano-convex lens (costing about one 6 inches in diameter)fixed to the under side of theaperture, so as to re-convergethe rays on the focusing effects can, however, beprocured without this with agood lime-light, in spite of whatis then wasted. In any case,all but the condensers are re-moved from the lantern, andthese are so adjusted, and thebox so placed in front, that the field is just covered andno more. To one side of the box is fixed a stout perpen-dicular brass rod or tube, the same gauge as the pillar-standsalready described, and furnished with two sockets like Fig. lower socket bears a focusing lens, either plain orachromatic—it may be the one already described (Fig. 7)—and the upper socket the plane reflector, which reflects the. Fir,. 13.—Vertical Attachment. 1.] VERTICAL ATTACHMENT. 21 image to the screen. These two sockets must, of course, befitted to the apparatus, as it is necessary that the lengthsB c (Fig. 6) should in this case be such as to bring the lensand reflector central with the perpendicular axis; but theens and mirror already described will answer for the rest per-fectly well. It might be supposed that the first surface ofthe mirror would cause a double image on the screen; butit is so faint in comparison with the other, that any sucheffect is rarely perceivable at all, and only then to close andspecial observation. Besides its other more strictly experimental uses, thevertical attachment is very effective for projecting on thescreen extempore diagrams, which are easily traced on asheet of prepared glass laid horizontally. It often makes ademonstration much clearer to work out a somewhat com-plicated diagram step by step in t
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Keywords: ., bookcentury1800, bookdecade1880, bookidcu3192403121, bookyear1882
