Which scientist first discovered that white light is a mixture of a rainbow spectrum of light rays?

Possibly using a dispersive glass prism, the spectrum is often attributed to Newton. I think he may have written something (in Latin) called "Optiks" about it.

Newton understood that white light can be separated into its components because each ray of colour is deviated by the glass of the prism by a different amount. He realized, for example, that red light is consistently less deviated than violet light.