464 



CONSTRUCTION OF PRISMS. 



board opposite to the refracting edge. The hole for the handle 

 should be bored with a stout gimlet, but not so deep as to reach the 

 hole for the prism. The handle may be fixed with glue ; but if the 

 hole should have been made too deep, sealing wax must be used for 

 fixing it. A hole , bored into one of the triangular sides by 

 means of a fine gimlet or burnt . through with a hot wire, serves 

 for filling the interior afterwards with water ; this aperture should 

 not exceed 2 mm in width. The refracting surfaces of the prism are 

 formed by thin plates of glass fixed upon the sides of the wedge with 



FIG. 264 (an. proj., | real use}. 



sealing wax ; the plates may either be cut square by a glazier or pre- 

 pared with pastille, cutting them first round and then grinding off 

 the edges. First place a thin layer of sealing-wax, which may be 

 softened over the lamp, upon the rim of the wedge ; then hold one 

 of the glass plates with a pair of crucible tongs, as in fig. 264, over 

 the spirit-lamp, moving it cautiously to and fro until it is hot enough 

 to melt sealing-wax when touching itj and press it upon the layer of 

 wax 011 the wedge. When one plate is fixed and has cooled, proceed 

 with the other plate in the same manner. The plate must be heated 

 slowly and not too much, and care should be taken to place it at 

 once in the proper position upon the side of the wedge ; otherwise 

 the part through which the light has to pass will become partially 

 smeared over with sealing-wax, which cannot be removed again 

 after the second plate is fixed. All particles of wood which may 

 adhere to the sides after the boring and filing must also be removed 

 previously, as they cannot be easily taken out afterwards through 

 the aperture a. The water is introduced by means of a glass tube 

 about 15 cm long and as thick as a pencil ; one end of it is drawn 

 out long and thin, and the tube is then filled like a pipette by suck- 

 ing ; the point being inserted into the aperture the water is blown 

 into the interior of the prism and, the operation repeated until it is 

 full. The small aperture must be directed upwards while this is 



