ed sectional views for describing the operation of the variable focal-length lens shown in FIGS. 5A and 5B; FIGS. 7A and 7B are other enlarged sectional views for describing the operation of the variable focal-length lens shown in FIGS. 5A and 5B; FIGS. 8A and 8B are a plan view and a sectional view of the structure of a horizontal deflection section in the three-dimensional display shown in FIG.