Researches of Huggins, Secchi, Wolf, and Rayet–Spectra of different comets : bright bands upon a continuous luminous ground–Analysis of the light of Ooggia's comet in 1874–Chemical composition of different nuclei and nebulosities.

Physicists, it is well known, recognise three orders of spectra as produced by sources of light when a luminous beam emanating from these sources has been decomposed in its passage through a prism or a system of prisms.

A spectrum of the first order consists of a continuous coloured strip, exhibiting neither dark lines, nor bright bands separated by dark intervals ; it is, in fact, the solar spectrum, more or less brilliant in colour, and of more or less extent, but destitute of the fine black lines which belong to the spectrum of the sun. Incandescent solids or liquids produce these continuous spectra. Spectra of the second order are those which arise from sources of light composed of vapours or incandescent gas; they consist of a greater or less number of lines or brilliantly coloured bands, separated by dark intervals; the number, the position, and consequently the colours of these lines or luminous bands are characteristic of the gaseous substance under ignition. Every chemically simple body, every compound body which has become luminous without decomposition, has a spectrum peculiar to itself.