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OPTICAL PROJECTION 



the lecturer himself, if standing near, can do it with a touch 

 of his hand. The blank screen is never shown. A single 

 lantern is often fitted with an arrangement for gradually 

 darkening down to complete obscuration in front of the 

 objective ; and in that case also this carrier is best, because 

 in one moment, with no possibility of hitch or mistake, the 

 slide can be changed while the screen is dark, and the new 

 one will then emerge out of the darkness. This simple effect 

 is thought by many to be quite as pleasing as the now familiar 

 dissolving views. 



This carrier cannot unfortunately be used with two lanterns 

 side by side, for obvious reasons. In that case perhaps the 

 best form is that shown in fig. 75, where the inner frame 



FIG. 75. Carrier for Pair of Lanterns 



travels to and fro in the same way, but can only be in position 

 when pushed in, and has to be drawn back and out for the 

 slide to be changed. 



There are other carriers, some patented ; and any optician 

 with a large lantern business can show an assortment, or at 

 least representations of such from catalogues ; but the above 

 are in my opinion the best. 



76. Mechanical Slides. These are of many sorts, and can 

 only be mentioned here, a very large variety being merely of 

 a comic character. They seem still to maintain their 

 popularity amongst certain audiences, and are worth at least 

 enumeration, as the effects produced may at any moment 

 furnish some useful hint for more serious purposes. (See for 

 an example Chapter XV.) 



