Possible Clues:

• "Dark Side of the Moon" cover has one
• Po-faced about school figure mixing up colours
• Solid figure of knight turning up to be received by premier
• Teacher turning up during autopsy's a possible shedder of light
• Refracting body
• Elevated teacher installed in fairly quiet block for light work
• It separates light into colour spectrum
• Light shower?
• Optical device used to deviate a beam
• Figure purse contains shilling