Modern improvements in optical methods lend additional interest to an examination of the causes which interfere with the absolute homogeneity of spectrum lines. So far as we know these may be considered under five heads, and it appears probable that the list is exhaustive:

1. (i) The translatory motion of the radiating particles in the line of sight, operating in accordance with Doppler's principle.

2. (ii) A possible effect of the rotation of the particles.

3. (iii) Disturbance depending on collision with other particles either of the same or of another kind.

4. (iv) Gradual dying down of the luminous vibrations as energy is radiated away.

5. (v) Complications arising from the multiplicity of sources in the line of sight. Thus if the light from a flame be observed through a similar one, the increase of illumination near the centre of the spectrum line is not so great as towards the edges, in accordance with the principles laid down by Stewart and Kirchhoff; and the line is effectively widened. It will be seen that this cause of widening cannot act alone, but merely aggravates the effect of other causes.

There is reason to think that in many cases, especially when vapours in a highly rarefied condition are excited electrically, the first cause is the most important. It was first considered by Lippich and somewhat later independently by myself.