A manganese-activated zinc silicate phosphor according to the present invention satisfies 0<\u03b2\u2212\u03b1\u22660. For this purpose, a pair of right and left intermediate cases are supported by right and left wheel supports of a vehicle body, each to be pivotable about a first steering axis extending substantially vertically. The inhibitor is soluble in water so that when the controlled-gelation aqueous solution contacts aqueous media the inhibitor disperses so that gelation occurs and the formation is blocked. The shoes are moved between retracted and extended positions by pneumatic cylinders.