Improvement on Newton’s original experiment
The spectrum formed in Newton’s first experiment was impure. This resulted from the fact that it consisted of a series of circular coloured images all overlapping. Later Newton devised the arrangement shown in Fig. 25.2, which produced a fairly
pure spectrum. A converging lens L is placed so as to form an image I of a narrow slit S brightly illuminated by white light. A prism is then placed in the path of the light from the lens. Dispersion of the light on passing through the prism leads to the
formation of a spectrum VR on a white screen. This consists of a series of coloured images of the slit all touching one another. If the slit is made narrow, overlapping is reduced to a minimum and the resulting spectrum is fairly pure.