Modern magic lanterns; a guide to the ... - Yesterday Image

90 MODEEN MAGIO LANTEBNB.appertaining to a lantern by some long Greek name, are oftencalled Aphengescopes. It is a useful device occasionally fordemonstration purposes, but owing to the very great lossof light, can only be employed with limelight or with theelectric arc, and then is never Very satisfactory. In Fig. 65the source of light is shown at b, and the condensers atCO, the object, which is placed at d, having its imagufocussed upon the screen by means of the objective, e.There is no need to get a special lantern for the purpose,as a box is easily arranged to cEtrry the objective at one endand to receive the nozzle of the lantern in the directionshown. Such a box should have its interior lined with blackpaper, or better, with black velvet. The back can be arrangedto carry the objects it is desired to exhibit, accessbeing obtained by the side a, which is closed with a curtain.When the Aphengescope i,s being used, and, indeed, in alarge number of other cases, it will be found that, unlesssteps are taken to prevent it, harm will very frequently occurfrom delicate instruments or inflammable substances beingexposed to the intense heat concentrated upon them by thecondenser. To obviate this, what is known as an alum cellis employed. This is an arrangement similar to that shownin Fig. 61 ; in fact, the same cell can be employed foreither purpose, as required. It is filled with a cold saturatedsolution of alum, which must have been carelully filtered soas to be quite free from dust or other floating particles. Thesolution will in course of time get fairly hot, but while allowingnearly all the light to pass, it will absorb the heat, whichwould otherwise be doing harm. In the absence of the alumsolution, plain water can be employed. It is not quite sobut is satisfactory enough for most purposes.efficient,Another arrangement of the lantern for demonstrationpurposes is that shown diagrammatically in Fig. 66, asemployed for vertical projections, as they are called. Thebeam of light from a, as it leaves the condenser, b, fallsupon a mirror, fp, placed at an angle of 45 degreeswith the horizontal, from which it is reflected verticallyupwards. The table, d d, upon which the object to beprojected is placed, is immediately over the mirror, whileabove it is placed the objective, e, bearing above it again a