Model of one clear shell. The clear shell is modelled by a pure sphere (central blue sphere in the figure; the width of the shell is not represented for sake of clarity). The projection of the shell on the sky will always appear as circular, independently of the galaxy inclination (represented by the circle projected on the plane on the sky, between the observer and the modelled shell). The density profile obtained by collecting the light through a slit placed in front of the circular projection and encompassing its centre is also represented (see the projected 2D profile in the bottom of the figure, placed here also for sake of clarity).