176 



OPTICAL PROJECTION 



freedom of movement and of access being required than is 

 needed, in the mere projection of slides. Very often, unless 

 the lantern itself is a revolving one, it must be readily capable 

 of being turned about at various angles with the screen. 



A simple and effective way of working is to place lantern 

 and accessories on a bare and smooth wooden table, whereon 



each lens or other piece of ap- 

 paratus can be adjusted ad 

 libitum. Such a table should 

 never be covered with cloth or 

 baize, as this would destroy all 

 freedom of movement ; but if 

 appearance be desired, it can 

 easily be draped round, and its 

 surface dead-blacked or ebonised. 

 If the condensers used for 

 physical work are of the proper 

 height, the lantern needs no 

 more than a circular base, on 

 which it can turn easily, to be 

 clamped in any position by a 

 screw under the table. The 

 heaviest lantern maybe mounted 

 so as to turn easily, by letting 

 a revolving metal plate carrying 

 the lantern, with a shallow 

 groove in its under surface run 

 upon a few bicycle balls. 



Otherwise it is convenient for the revolving base to be 

 mounted on three short legs, so as to form a short tripod, 

 which stands upon the table. Each leg should in this case 

 have a piece of india-rubber cemented on the bottom, so that 

 the tripod may ' bite ' and remain steady. The subsidiary 

 apparatus may still be placed about the table as before, if the 

 heights are adjusted for such an arrangement. But it is more 



Flo. 99 



