Long-slit spectroscopy can also be used to observe the expansion of optically-thin nebulae. When the spectrographic slit extends over the diameter of a nebula, the lines of the velocity profile meet at the edges. In the middle of the nebula, the line splits in two, since one component is redshifted and one is blueshifted. The blueshifted component will appear brighter as it is on the "near side" of the nebula, and is as such subject to a smaller degree of attenuation as the light coming from the far side of the nebula. The tapered edges of the velocity profile stem from the fact that the material at the edge of the nebula is moving perpendicular to the line of sight and so its line of sight velocity will be zero relative to the rest of the nebula.