and we see that the second term indeed gives the negative scattering length density of the 'hole' left in the shell. Since the local density of particles in the shell will differ slightly from the bulk solvent, the first term of the form factor needs to reflect the change in scattering contrast between these two (albeit not very well-defined) regions of solvent, ρ s -ρ bulk , as is indeed the case in eq. 21. The model is illustrated in the top right part of fig. . We calculate the scattered intensity as