110 



OPTICAL PROJECTION 



is not at all unusual to make them so that the top one can be 

 separated, and placed by the side of the other, to use with 

 oil lamps. In any case the two doors should be separate. 

 It saves a sovereign to have doors only on one side of the 

 lantern; but it is very much better to have them on both 

 sides, as it is impossible to tell in what corners or positions 

 the lantern may have to be worked. To make the two discs 

 coincide upon the screen, each of the metal plates carrying a 



FIG. 62. Bi-unial Lantern 



front is hinged, so that the two optical systems may be 

 inclined to each other and adjusted by screws, any slight side 

 adjustment necessary being made in the stages. The inclined 

 front carries the condenser and slide as well as the objective, 

 so that the whole is kept in one optic axis. 



The bi-unial is much the most convenient form of double 

 lantern, as the slides can be manipulated freely, and if neces- 

 sary by two persons, or by a hand at each end. It is also 



