I7S OPTICAL PROJECTION 



In theatres it is seldom convenient to work upon a table, 

 which would occupy too much space in the area, and be 

 awkward to move about, as may be often needed. \ 7 ery often 

 it is more convenient to have two or three detached single 

 lanterns than a combination ; and the lecture table is required 

 for other and general purposes. The lantern is then best 

 mounted upon a strong wooden tripod, more or less resembling 

 fig. 100, with a steady adjustment for raising and lowering. On 

 such a tripod should be mounted a revolving top similar to 

 fig. 99, furnished with an adjustable wooden pillar to support 

 the end. This is the method of work chiefly adopted in the 

 Royal Institution. 



91. Light for Working. If an arrangement is at hand, as 

 in the Institution just named, whereby all the lights in the 

 auditorium can be turned up or down in- 

 stantly and without mishap, nothing can be 

 better, except for the one point immediately 

 mentioned. But more homely appliances 

 must often suffice ; mid it is distinctly bette? 

 for experiments which really tax the illu- 

 FIG. ior minating power of the apparatus, if the light 

 turned up be only one gas-burner sufficient 

 for the lecture table, inasmuch as the eyes are more sensitive 

 and able to observe phenomena, when not allowed to perceive 

 any bright light between. 



An Argand burner, shaded as in fig. 101, will be found very 

 effective in this way, shading the direct light at all times both 

 from the screen and the audience, while giving perfect light 

 when desired. When turned down to a rather small blue flame 

 not in the slightest danger of going out, it will give an ap- 

 preciable dim light over the apparatus, while leaving the room 

 and screen practically in darkness. It can be mounted on 

 either a bracket, or a fixed or movable pillar-stand. 



Another most excellent lecture appliance is a burner 

 known as the matchless or self-lighting burner, which turns 



