"e plane as that of FIGS. 4a and 4b, while FIG. 40 is a sectional view along the I v plane perpendicular to the axis OP. The embodiment of FIGS. 4c and 4d utilize mirror reflecting surfaces 5 and 6 that are relatively inclined as compared to the embodiments disclosed in FIGS. 4a and 4b." . . . . .