A stress-optical relationship has been derived for colloidal suspensions which enables the use of optical dichroism measurements to distinguish between thermodynamic and hydrodynamic contributions to the stress tenser, Through theological and optical measurements on suspensions of model, hard-sphere silica particles, we verify the stress-optical relationship and test current microstructural theories and simulations. Specifically, the 45 degrees dichroism in the flow/gradient plane is found to be proportional to the thermodynamic contribution to the shear stress at all volume fractions studied. When appropriately normalized, this constant of proportionality is shown to be relatively insensitive to particle size and volume fraction, and is in agreement with an "exact" low-shear, dilute-limiting calculation. Three microstructural theories valid at finite concentrations are shown to deviate substantially from the measurements at higher concentrations. Using the stress-optical relationship, we show that the shear thinning of these suspensions is attributable to changes in the thermodynamic forces’ contribution to the stress, consistent with both theory and simulation. Also, we provide evidence that shear thickening is attributable to increased hydrodynamic interactions, which supports the view that shear thickening results from strong lubrication forces generated by the formation of nonpermanent clusters of particles.