Newton's  way of approaching the question

The first astronomical telescopes at the beginning of seventeenth century were impractical long refractor telescopes that, instead of giving pinpoint images of distant stars, gave blurry images with colorful fringes owing to chromatic aberration. Newton had invented an ingenious telescope design using a reflecting mirror which had no color aberration, and this he wanted to present to the Royal Society with his new theory of light.

It was this pinpoint image of a star that Newton had in mind when he arranged his first experiment with a prism, letting sunlight shine through a small hole in his window shutter. When he placed a prism in the way of this narrow beam of light, it produced the familiar continuous oblong spectrum on the opposite wall.