Scattering due to the sample as a whole

In the case of an isotropic material for which I(s) depends only on the magnitude of s, Equation (1.85) can be written as 

Let us designate the mean value of p(r) throughout the sample by (p) and the deviation of p(r) from its mean by 

The third integral in (1.90) is zero by the definition of q(r). The fourth is also zero except when r is of the same order of magnitude as the sample dimension so that the effect of the edge of the sample becomes important. Since we are not interested in an effect that becomes noticeable only when r is of macroscopic dimension, the fourth term in (1.90) can be neglected. Thus (1.90) reduces to 

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