DUL \T.I:V FORCES. LIGHT. 



MXJPE8. 



lidcr pertly inserted into the holJiT ; c i* part of ihe 



Fif 43. 



rack-work for adjusting the lens to the picture. The 

 adjustment is regulated by the appearance of the picture 

 on the screen, d d is the din.-- n-ndts; that is 



to say, the apparatus for making one picture gradually 

 disappear while another gradually comes into view before 

 the spectators. This change of pictures is managed 

 thus : The bar/, which is attached to the screen d e d, 

 ascends and descends in a collar ; this is screwed to the 

 front of the box on which the apparatus is mounted. It 

 i provided with a rack, which is acted on by a pinion 

 marked g, and the pinion is turned by a handle. Sup- 

 posing the pair of lanterns to be in action, and each 

 lantern to contain a slider, one of these sliders will give 

 n picture upon the white screen, while the other is 

 itopped by the upper limb of the screen d. If, then, the 

 screen d e d is gradually raised, by the turning of the 

 handle h, which causes tho racked bar / to ascend, the 

 acting slider will be gradually covered by the rising 

 comb of one end of the dissolver, while the other will, in 

 the same proportion, become uncovered by the rising 

 comb of the other end of the dissolver; so that, when 

 the first picture is quite obscured, the second will be 

 quite developed. In that manner, tho first picture is 

 made to dissolve or gradually fade into the s< 

 picture. At e, in Fig. 43,. the dissolving screen, d d, 

 has a hinge, the use of which is to admit of suddenly 

 cutting off the action of the picture in the corresponding 

 luiiern. This action is required for the exhibition of 

 ' i ling and other sudden effects. The change of sliders 

 in the lanterns is made when each nozzle is alternately 

 obscured. The turning of the rack-work, by wlu'ch the 

 dissolving is effected, must be slow, regular, and noise- 

 less. 



\Ve have here given the plan usually employed in the 

 dissolving-view lantern*. We, however, will give another 

 one, which we used ourselves in the lanterns of tho 

 Glasgow Polytechnic, which is extremely simple in its 

 conitruction. (See Fig. 44). 



No. 1 is a piece of wood with holes, which are placed 

 exactly opposite to the ends of the lantern lenses. 



No. 2 is a slip of wood made to slide at the back of 



N'-. 1, also having holes, but in different positions. It 



Hunt that only one of the holes, as cut in this, can 



be open at the same time with those of No. 1. Thus 



those on tho right hand are open ; but supposing that 



Fig. 41. 



o o 



No. 1. 



O 



o 



No. 3. 



the slip is moved to the ri^-lit hand, then tin- 



will 1" ud the left one opened. Thus l.y sliding 



No. 2 to and fro at the back of Iv'o. 1, tho views are 



dissolvril. 



Tliis arrangement is an exceedingly useful and eH'ertive 

 one; and any person having a pair of lanterns, ,-an easily 

 arrange them as dissolving-view apparatus, by having 

 two slips cut as ilhistrated in the engraving, and by 

 ii\:n,' them in a frame in front of the nozzles of the 

 lanterns. 



In arranging tho lanterns, they must be placed at a 

 angle with each other, so that tho light they east 

 on the screen shall fall equally on it. In other words, 

 the centre of each picture, cast by each lantern, must 

 fall on the centre of the screen : this is technically termed 

 "centre-ing" the lanterns. 



Much care is required to focus the lenses, so that a 

 clear and distinct picture be obtained. As, however, 

 this can only be arrived at by practice, we forbear giving 

 special directions on the subject, except so far as to 

 state, that the pictures should be so focussed as that 

 their centres may present the best possible image on the 

 screen. 



It will be beyond our plan to give any directions in 

 reference to painting the slides, as that belongs to the 

 art of the glass painter. An unlimited supply, either- on 

 hire or for purchase, is kept of all kinds, whether views, 

 astronomical diagrams, chromotropes, <tc., by most 

 opticians in our large towns. Chromotropes are simply 

 two pictures so arranged that they may revolve over 

 each other on a common centre ; and the lines painted on 

 the glass thus intersecting each other, produce a very 

 pleasing optical effect. 



In concluding these directions and remarks, we would 

 mention, that the magic lantern and dissolving view 

 apparatus, may be made an excellent medium for the 

 instruction of youth ; and they certainly should be an 

 adjunct to the means of communicating knowledge to 

 every public school. We shall not, however, enlarge 

 here on this subject, because we propose to do so when 

 describing the construction and arrangement of the oxy- 

 hydrogen microscope. 



The physioscope is a modification of the magic lantern ; 

 but instead of the light being transmitted through the 

 object whose imago is to be represented, a powerful 

 light is cast thereon : the reflected rays are collected by 

 lenses, as in the magic lantern, and the magnified i 

 is cast on the screen. The human face may thus be 

 presented in a highly magnified state; as, indeed, any 

 other solid object may be. Tho difficulties of the ar- 

 rangement are, however, very great ; and the comparative 

 rarity of its use will not demand from us a more 

 detailed account of its construction. 



MICROSCOPES. 



ONE of the most popular of all optical instruments is the 

 microscope ; and perhaps throughout the whole of the 

 diilereiit applications of the laws of the refraction of 

 light, we cannot discover one to which we are so indebted 

 for a knowledge of the beauties and wonders of natural 

 history, and the varied appearances of almost every kind 

 and form of matter. We shall first describe a consider 

 able variety of microscopes, commencing with the most 

 sin.ple kinds, and passing to those of a compound and 

 achromatic form ; and afterwards illustrate tho arrange- 



