DISSOIVINO VIRWS, ETC.] 



UXDULATORY FORCES. LIGHT. 



the nrranyonient. 5th. When india-rubber bags are 

 employed, the weights pressing each should be equal on j 

 luith bags. This will ensure a uniformity of supply. In 

 larye institutions, gasometers or gas-holders are always 

 used, and these are sometimes of considerable size. They 

 are, however, unfitted for private use, for which nothing 



r than the bags to which we have alluded. 

 In Vi;. 4'_>, a very convenient form for a small gas- 

 p\t. 4. holder is illustrated. It consists of a 



trough to hold a supply of water, 

 from which runs a pipe fitted with a 

 stopcock, into the bottom cylinder 

 which is intended to contain the gas. 

 ( )n the right-hand side is a cock, by 

 s of which the gas passes when 

 the holder is in use. When such an 

 arrangement is to be filled with gas, 

 all the cocks are first to be opened, 

 and water must be poured into the 

 trough. This will run into the cylin- 

 der and fill it. Each cock is then 

 closed, and a screw, observed at tho 

 left hand of the lower part of the holder, U removed. 

 If the vessel be quite air-tight, only a few drops of 

 water will escape. Into the opening thus mode, the 

 end of a retort from which oxygen is being made, is 

 introduced, so that the gas may pass into the holder. 

 The water will escape as the gas passes in, and it is 

 therefore advisable to place the vessel over a tub during 

 1 rocess of filling. Of course, two gas-holders will 

 be required for the purposes we have referred to one 

 for oxygen, and another for hydrogen. The latter is 

 by passing in the tube from the hydrogen bottle ; 

 or, if coal gas be used, an india-rubber pipe may be 

 attached to the stopcock on the side, the other end 

 In-ill;,' connected with an ordinary gas jet The holder 

 must first be filled with water, and the lower screw 

 afterwards removed, as when filling with oxygen. 



In public institutions, where large gas-holders are nsed, 

 a sufficient quantity of gas is prepared to last for a few 

 day* ; and the slight admixture of air, through accidental 

 leakage or otherwise, which always occurs, is a matter 

 neither of importance or of danger. In using small 

 hoi, lent, such should be first entirely emptied of all gas 

 by means of water, and then they may be refilled at a* 

 short a time as possible before their contents are required 

 for use : an equality of pressure should also be main- 

 tained in each, so that the gases shall pass from their 

 respective vessels at an equal speed and pressure. A 

 stock of lime cylinders may easily be made by means of 

 a file and some good hard lime ; and that obtained from 

 nd is best for the purpose. The cylinders, which 

 may average an inch and a-half in height, and three- 

 quarters in diameter, should have a hole pierced half- 

 way, or wholly through them, to permit of their resting 

 on tho wire-holder which sustains them before the jet ; 

 and until required for use, they must be kept wrapt up 

 in paper, and in a glass-stoppered or tightly corked bottle, 

 to prevent access of moisture, which would speedily 

 : . >y them. As they are of next to no cost if made 

 at home, a fresh one should be used on each occasion. 

 If purchased of tho optician, they become rather an 

 expensive affair ; and yet these have no advantage, in any 

 respect, beyond such as may be made by any person in 

 the manner we have explained. 



Having thus given sufficient directions respecting the 

 management of the light, we proceed to speak of tho 



u\' ami general care of the lenses. 

 Most persons attempting to show views by means of 

 any of the instruments to which we are now direct- 

 ing attention, have to experience the annoyance of 

 finding the lenses covered with moisture at certain 

 seasons of the year a circumstance which almost entirely 

 prevents the appearance of the picture on the screen. 

 This partly arises from the vapour evolved by the com- 

 bustion of the materials from which the light is pro- 

 duced ; indeed, water is formed in abundance when an 

 r gas flame, and the lime light, are employed. This 

 evil is, however, easily remedied by having the light 



lit for some time before the exhibition takes place ; and, 

 in the case of the lime light, by allowing the hydrogen to 

 burn gently against the lime cylinder, and thus drying it 

 also at the same time. In every case the lenses should 

 always be a little warmer than the air of the apartment ; 

 and thus, of course, all chance of the condensation of 

 moisture is at once removed. It is a good plan to cover 

 the whole of the lantern by means of a thick sheet of 

 baize, so as to prevent the deposition of dust or dirt on 

 the lenses when they are not in use. 



The lime light causes a considerable quantity of volatised 

 lime to settle on the lenses, and soon dims their surface. 

 This requires that they should be frequently cleaned. 

 Unless care is taken tn this operation, tho polished 

 surfaces soon become scratched, and of course are so 

 far deteriorated. The plan we have always adopted, is 

 that of first removing all the lenses from their seats, and 

 wiping each perfectly with a damp old linen towel, to 

 take off the "rough coat" of dirt. Each lens should 

 then be carefully washed, by means of cotton wool, with 

 spirits of wine (not methylated spirits, because these 

 contain a resin in solution), and afterwards it may be 

 polished with a soft chamois leather, or what is still 

 better, an old silk handkerchief. No polishing powder, 

 or anything of a solid nature should be used, because 

 these almost invariably contain gritty hard matter, which 

 soon spoils the glass. The towels should be kejit in a 

 place where no dust or dirt can settle on them. Almost 

 every one has an idea that nothing is more difficult than 

 to scratch glass ; but chemists, opticians, and photogra- 

 phers know, to their cost, that the difficulty lies in pre- 

 venting, rather than in producing, that result. The 

 lanterns should afterwards be well cleansed from all dust, 

 both inside and out ; and the lenses may then be replaced 

 in the exact position which they hod previously occupied. 

 The slides or views should also be kept scrupulously 

 clean, or ugly black spots of dust may appear on the 

 screen; and when not in use they should be kept in 

 nicked shelves, or in boxes in which they may be kept 

 separate, and thus any risk of fracture be avoided. 



U e have thus given minute directions in reference to 

 the general management of the magic lantern, Arc. ; and 

 may here remark, that the only difference between it and 

 the phantasmagoria, is, that the former is so used as to 

 show the pictures on the same side of the screen as the 

 spectators are placed; whilst, in the hitter, the views 

 are cast on a transparent screen, and the lantern is on 

 the side opposite to the position of the spectators. By 

 this arrangement the size of views is diminished by 

 bringing the lantern nearer to the screen ; and increased 

 by removing it therefrom. This is an advantage when 

 the instrument is employed to amaze young folks, who 

 are greatly puzzled to know whore the views come from ; 

 and also at the sight of the figures apparently retreating 

 or advancing. An opaque screen or thick linen sheet is 

 best for the magic lantern ; whilst a thin calico sheet, 

 moistened with water, answers best for the exhibition of 

 the phantasmagoria. In either case the screen should 

 be kept tightly stretched at each side, and at the corners, 

 and should be in such a position that draughts of air may 

 not cause it to swing to and fro. 



DISSOLVING VIEWS, AND CHROMOTROPES. 



AT one time the exhibition of dissolving views was mn- 

 fined to the London Polytechnic Institution ; and the} 

 have invariably proved a most interesting and instructive 

 addition to entertainments uniting science and amuse- 

 ment. 



Two magic lanterns placed side by side, and tlirowing 

 an equal-sized picture on the screen, are the apparatus 

 required. There are additions, which we proceed to 

 describe, by means of which the "dissolving" effect is 

 produced. All our previous remarks on the magic 

 lantern are therefore equally applicable to this arrange- 

 ment in every other respect. The following engraving 

 illustrates a very usual form of the dissolving view ap- 

 paratus. 



In Fig. 43, letter o represents the slidcr-holdei ; b is 



