[Illustration: FIG. 9.—Prism spectroscope. To the right is seen the collimator, to the left the telescope, in the foreground a scheme for illuminating the cross-wire. (From an old print.)] In Newton’s original experiment the different wave-lengths were but imperfectly separated. A spectrum with pure wave-lengths can be obtained with a _spectroscope_ (cf. Fig. 9). The light to be investigated illuminates an adjustable vertical slit in one end of a long tube, called the collimator, with a lens in the other end. If the slit is in the focal plane of the lens, the light at any point in the slit goes in parallel rays after meeting the lens. It then meets a prism, with vertical edges, placed on a little revolving platform. The rays, refracted by the prism, go in a new direction into a telescope whose objective lens gives in its focal plane, for every colour, a clear vertical image of the slit. These images can be examined through the ocular of the telescope; but since the different colours are not refracted equally, each coloured image of the slit has its own place. The totality of the slit images then forms a horizontal spectrum of the same height as the individual images. By revolving the collimator different parts of the spectrum can be put in the middle of the field of view. To facilitate measurements in the spectrum there is in the focal plane of the collimator a sliding cross-wire with an adjusting screw or a vertical strand of spider web. [Illustration: FIG. 10.—The mode of operation of a grating. _A_, grating; _C, D, E_ ... _H_, slits; _M M_, incident rays. When _D D′, E E′_ ... are a whole number of wave-lengths, the light waves which move in the direction indicated by _C N_ and are collected by a lens, at the focal point will all be in the same phase and therefore will reinforce each other. In other directions the light action from one slit is compensated by that from another.]