Spectroscope: Airy’s Prismatic Spectrum-Apparatus (1862)

This instrument was first mentioned by Airy in his 1862 Report to the Board of Visitors, where he wrote:

‘I have prepared a Prism-Apparatus, to be used in conjunction with the S.E. Equa­toreal [the 12.8-inch Merz Refractor], for examination of the spectra of fixed stars; but hitherto I have been able to do little more than adjust its parts.’

In his report for 1863 he elaborated:

‘It is constructed on the principle of giving breadth to the linear spectrum by allowing the conical pencil of light, that diverges from the image of a star, to fall in a diverging state upon the prism, which is placed in a position differing from that of minimum deviation. When both these conditions are secured, the light, on emergence from the prism, diverges differently in the two transverse planes; and the apparatus of lenses which then receives the pencil, and which gives complete convergence in the direction that produces purity of the spectrum, does not give complete convergence in the direction that produces narrowness of the spect[r]um. The construction is the simplest that has been proposed for its purpose. Still I regard it as experimental; there may be some risk in the oblique refraction of conical pencils; and I propose soon to try the effect of the prism with parallel pencils of rays passing through it; breadth being given to the spectrum by a cylindrical lens.’

And In 1864 added:

‘The Prismatic Spectrum-Apparatus has been altered in the manner described in the last Report. Achromatic object-glasses are placed on both sides of the prism, so that each pencil of light through the prism consists of parallel rays; and breadth is given to the spectrum by a cylindrical lens. The spectral lines are seen straighter than before, and generally it is believed that their definition is improved.’