Isaac newton and the composition of sun light
The story of solar, and hence also of astronomical spectroscopy, began in 1666 when the young Isaac Newton (1642–1726) wrote these famous words:
I procured me a Triangular glass-Prisme, to try therewith the celebrated Phaenomena of Colours. And in order having darkened my chamber, and made a small hole in my window-shuts, to let in a convenient quantity of the Suns light, I placed my Prisme at his entrance, that it might thereby be refracted to the opposite wall. 
The quotation is from Newton's first paper, which he sent in 1672 to the Royal Society. As he himself notes, the phenomenon was already well known and views on the nature of colour by Descartes, Grimaldi, Hooke and others had already been published. The key feature distinguishing Newton's repeat of the experiment was probably his large distance from the prism to the screen or far wall . Newton tells us he used a wall 22 feet from the prism, so allowing sufficient space for the colours to separate out and to give a clear spectrum. ‘Comparing the length of this coloured spectrum with its breadth, I found it about five times greater; a disproportion so extravagant, that it excited me to a more than ordinary curiosity of examining, from whence it might proceed’ .