Amici prism

The prism angles and materials are chosen such that one wavelength (colour) of light, the centre wavelength, exits the prism parallel to (but offset from) the entrance beam.

Other wavelengths are deflected at angles depending on the glass dispersion of the materials.

[1][2] This doubling of the original prism increases the angular dispersion of the assembly and also has the useful property that the centre wavelength is refracted back into the direct line of the entrance beam.

Amici himself never published about his nondeviating prism, but rather communicated the idea to his friend Donati, who constructed the device for observations of stellar spectra.

[3] Donati's publications of his observations (in 1862) were the first disclosure of the prism doubling idea, and because the prism was practical to build and much more compact than multiple-prism arrangements typical in that period for producing high spectral dispersion, Amici's invention quickly caught the attention of researchers throughout Europe.