THE MAGIC LA-VTERN.] 



UNDULATOtfY FORCES. LIGHT. 



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section ; and as they are all constructed on similar 

 principles, we have deemed it desirable to treat on them 

 collectively. 



In all, except the physioscope, the object is strongly 

 illuminated by light from a lamp or other arrangement, 

 and the rays passing throuyh the picture are subsequently 

 magnified, and the image received on a white screen. 

 In the physioscope, the rays reflected from the object are 

 magnified, and their image received in a similar manner. 



Commencing with the magic lantern, with whose pretty 

 effects we are all so well acquainted, we may remark, 

 that those sold for the amusement of young persons, are 

 constructed on exactly the same principles as the dis- 

 sohing views employed in the larger arrangements of 

 public institutions ; and, indeed, only differ in their size 

 and quality of workmanship. 



The best form of an ordinary magic lantern is that of 

 two Bets of lenses, each of which generally consists of 

 two plano-convex lenses. The larger of these two sets is 

 I nearest the source of light ; and is called the con- 

 denser. This receives and condenses the light, which 

 then passes through the painted glass or other object ; 

 and is afterwards transmitted to the next set, whose 

 object is to magnify the rays so travelling. The relative 

 distance between the two sets of lenses can easily be 

 altered ; because, whilst the condenser is fixed, the mag- 

 nifying glasses are easily moved to and fro in a tube 

 by mean* of rack-work ; or, being in a tube which fits 

 . in that holding the condenser, they are easily 

 I by the lingers. This operation is called 

 <in,' ;" and on care, in this respect, depends 

 tlio accuracy of outline of the picture on the set 



We need scarcely say, that unless the axis or line 

 passing from each lens proceeds through each exactly 

 in their centres, the instrument will not give suc- 

 cessful results. The source of light must be so 

 placed, that its moat brilliant portion is also in 

 the s;une straight line. I.i other words, a hori- 

 zontal line passing from the centre of the light to 

 the outer lens, must pass exactly through the centra 

 of all tlio lenses. The following engraving illus- 

 trate!! the section of a magic lantern of the most 

 improved form. 



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In Fig. 38, a a represents the box of the lantern ; b, 

 the groove in which the painted slides are placed ; c, the 

 source of light ; d, the condensing lenses ; e, the magni- 



,' lenses ; /, a stop to cut off the superfluous rays, 

 which would diminish the brightness of the picture ; g g 

 represents an imaginary straight line, passing through the 



'-. of tin; light and of all the lenses; h is a nut, by 

 which the lenses are propi-rly focussed. In purchasing 

 this instrument, care should l>e taken that all the lenses 

 are free from streaks or hollow specks, as these mar con- 

 siderably the good effect of the picture on the screen. 

 In occasionally using the electric light, we have observed, 

 that the light passing from bod lenses presents almost a 

 mass of lines of shadow. The worst glasses, however, 

 rarely show this to any great extent on the screen when 

 ordinary artificial light U employed. 



Everything depends on a good strong light being em- 



ployed to illuminate the objects exhibited in any of the 

 arrangements to which we are now referring ; and this, 

 therefore, demands our attention before we proceed to 

 describe their separate details. 



For instruments of a small size, an oil lamp, or gas 

 flame may be employed ; but when large arrangements 

 are used, the oxy-calcium, oxy-hydrogeu, or electric 

 light becomes necessary. 



The oxy-calcium light is a very simple and useful con- 

 trivance ; by means of which, considerable illuminating 

 power may be obtained. It depends on the fact, that 

 almost any flame, on being urged by a jet of pure oxygen 

 gas on to a piece of burnt lime, will produce an intensely 

 white light. Various arrangements are sold by the 

 instrument-makers for this purpose. The oxygen is 

 produced from chlorate of potass mixed with half its 

 weight of block oxide of manganese. The mixture is 

 placed in a glass retort, to the stem of which an india- 

 rubber tube is attached, connecting the retort with a gas- 

 bag or bladder. On applying heat to the mixture, by 

 means of a spirit or gas lamp, abundance of pure oxygen 

 is afforded ; which passing into the bag, soon fills it. It 

 is advisable to allow the gas to run through some water 

 in a bottle, so as to prevent the passage of injurious sub- 

 stances, which may injure both the brass connections and 

 the texture of the bag itself. The following engraving 

 Fig. 39. 



Illustrates an apparatus of this kind ; which we adopt 

 ourselves for making oxygeu on a small scale, and for 

 filling a bladder. 



This arrangement is thus constructed and managed. 

 After tying a bladder to a stopcock, drive out all the air 

 it contains by gentle pressure, and leave it moist. Then 

 tie to the end of the stopcock a piece of india-rubber 

 tubing about two inches long. Into a wide-mouth bottle 

 holding about a pint of water fit a cork which luus had 

 two holes bored through it, just large enough to receive 

 two pieces of pewter gas-pipe. One of these must re;ich 

 to the bottom of the bottle, and the other should just 

 pass into its inside through the cork. The bottle, which 

 we will call the purifier, should then be three parts filled 

 with water ; and having the retort or flask ready for 

 making oxygen, attach its stem, by means of another 

 piece of india-rubber pipe, to the pewt r tube which 

 reaches to the bottom of the bottle. The short pipe is to 

 be attached to the ttojtcock oi the bladder. On the right 

 we have the Florence flask, in which the oxygen mixture 

 is contained ; in the centre the purifier, whoso object is 

 to retain any manganese or chlorine which may pass 

 over; and, on the left, the bladder is observed in the 

 process of filling. After all is prepared run a little 

 melted cobbler's wax over the cork of the bottle, to keep 

 it air tight; then open the stopcock of the bladder. On 

 lighting the spirit-lamp, the gas will pass from the retort 

 through the water in the bottle, and, proceeding through 

 the stopcock, will gradually fill the bladder ; as soon as 

 this is filled, remove the retort before taking the spirit- 

 lamp away, lest the water might pass into it and destroy 

 it. The above is a very simple and yet eil'ectivo arrange- 

 ment ; and its cost is trifling. 



Having thus obtained a supply of oxygen, the next 

 point of importance is its application to the required 

 purpose; and for this, a blowpipe and a lamp of some 

 kind are requisite. Almost every optical instrument- 

 maker manufactures these in what we may call a special 



