The assemblage of crystals constituting four of the quadrants as shown in Fig. 3 are securely cemented to a back plate 35 (Figs. 4, and 7) through theintermedi ation of a thin disk '36 (Fig. 7) of hard insulating material such as Bakelite, the disk being securely cemented to the back plate while the lower faces of the crystals, the filler blocks and the spacing strips are securely cemented to the