Isaac Newton didn't discover the spectrum itself. The spectrum of light, meaning the separation of white light into its component colors, had been observed before Newton.

However, Newton was the first to perform a systematic and detailed study of the spectrum, using a prism to split sunlight and then recombine it. He published his findings in 1672 in his work "A New Theory about Light and Colors."

So, while the spectrum itself was not a discovery of Newton's, his scientific investigation and explanation of the spectrum's properties and behavior were a significant contribution to our understanding of light.