Isaac Newton's prism experiments at Woolsthorpe

I had my "fan boy" trip to Woolsthorpe - as close as it comes to visiting a shrine in Physics! The top picture is the window that Isaac Newton used for his light experiments. A later owner enclosed that part of the room so these days light can't go as far as it did in Newton's diagram. I knew that he had used one prism to split the white light into a spectrum and another to pull it back into a white light. I was only vaguely aware of his other experiment: he took the spectrum and used a slit to take only the red light which he then put into a second prism. Only red light came out of the second prism. He was intent on disproving a theory then that light was colourless and that the glass itself gave the light all the colours of the spectrum. Each experiment shows that the glass is not adding colour. I need to read more about the nuances of why both experiments were considered necessary.