| http://www.w3.org/ns/prov#value | - The optical properties of the gel thus deviate from those of the surrounding material and gels have hitherto been detected using equipment comprising a light source and a light detector, the optical axes of which are displaced so that the detector does not normally see the point illuminated by the light source and only does so when something changes the light deflection, i.e. a gel appears so that
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