APPARA TUS FOR SCIENTIFIC DEMONSTRA TION 177 



convenient, as a rule, to prolong the revolving base as in fig. 99, 

 into a board about 9 inches wide and three feet or more in 

 length, A B, while another cross-board, c D, can be slid along as 

 shown in the section below the figure, so as to form a f -piece 

 of the same level, at any distance from the lantern within the 

 compass of A B. The use of this arrangement is very obvious. 

 Suppose we are projecting the surface of a soap -film. This 

 film will stand diagonally across 

 A B and reflect the light to the 

 screen approximately in the direc- 

 tion of c D ; and the transverse 

 board c D gives us a base on which 

 to place our focussing or projecting 

 lens. So of other cases in which 

 the lantern has to be deflected. 

 The end B of the board A B is of 

 course supported by an upright 

 of proper length, which may be 

 hinged to it. The revolving por- 

 tion of the stand can be quite well 

 constructed of seasoned hard wood, 

 if two layers are glued together 

 with the grain across, and there 

 will be little friction if a judicious 

 use be made of black-lead. 



When work is done upon a 

 table, there is one point to keep in 

 mind. If it is necessary to cant up the optical beam in order 

 to get a good position on the screen, it must not be done by 

 canting the lantern itself, as in slide exhibitions. The whole 

 table must be canted, by placing blocks under the front legs. 

 Otherwise all adjustments for height will be destroyed, as 

 apparatus is moved nearer to or farther from the lantern. 

 The optical beam used for physical work must be parallel to 

 the surface of the table. 



FIG. 100 



