. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. r a attached to a prong of the fork, the other prong beingsimilarly loaded. The reflected ray is re-reflected back to thescreen by the plane-mirror b, when the spot should be re- focussed to allow forthe extra distancefrom a to b, whichblurs the originalimage (this re-fo-cussing, or else anallowance for extradistance caused byreflections, shouldalways be attendedto in this class ofexperiments). Onnow exciting thefork, the spot is drawn out into a line of light; and ifthen the plane-mirror b
. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. r a attached to a prong of the fork, the other prong beingsimilarly loaded. The reflected ray is re-reflected back to thescreen by the plane-mirror b, when the spot should be re- focussed to allow forthe extra distancefrom a to b, whichblurs the originalimage (this re-fo-cussing, or else anallowance for extradistance caused byreflections, shouldalways be attendedto in this class ofexperiments). Onnow exciting thefork, the spot is drawn out into a line of light; and ifthen the plane-mirror b be slightly turned in its socket(which is better than holding it in the hand) the line isdrawn out into ripples cd, showing the single vibrationsseparately. Very fair ripples may be projected in this way. But theywill be much more brilliant if the pencil attachment (fig. 95)be employed, whereby much more light is sent through asmall aperture on the front. This aperture is then focussedby a loose lens. But the best use of this focussing lensdemands some consideration, according to the circumstances,. Fin. i:34.—Lissajous Method SOUND W and still more so in projecting compound figures to be pre-sently described. 141. Management of Acoustic Pencils of light is brilliant and the pencil nearly parallel beforefocussing, as from an electric lantern, it is generally more con-venient to first focus the spot on the screen, and then adjustthe forks or other vibrating apparatus beyond the lens, orbetween it and the screen. There will be abundance of light,more room for adjusting apparatus, and the figure will beprojected of the greatest size, which with forks is desirable. But in projecting reeds as presently described, the greatestsize may be undesirable; or light may be deficient. In thelatter case, more light can be passed through a small apertureby somewhat converging the rays upon it, which divergeagain. Suppose we use a pencil thus diverging somewhatfrom a £ inch ap
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