Shine out distinct adown the watery bow;
While o’er our heads the dewy vision bends
Delightful—melting on the fields beneath.
Myriads of mingling dyes from these result,
And myriads still remain.—Infinite source
Of beauty! ever blushing—ever new!
Did ever poet image aught so fair,
Dreaming in whispering groves, by the hoarse brook,
Or prophet, to whose rapture Heaven descends?”
The spectra which Newton obtained by admitting the solar beams through a circular aperture, were, however, not simple spectra. The circular beam may be considered as built up of flat and very thin bands of light, parallel to the edges of the prism, and a simple ray would be formed by one of these flat bands; as the round opening would allow an indefinite number of such rays to enter, each would produce its own spectrum on the screen, and the actual image would be formed of a number of spectra overlapping each other. When the aperture by which the light is admitted consists merely of a narrow slit, or line, parallel to the edges of the prism, we obtain what is termed a pure spectrum. When the prism is properly placed, an eye, viewing the fine slit through it, sees a spectrum formed, as it were, of a succession of virtual images of the slit in all the elementary coloured rays.