Channelled spectra with a speckle interferometer consisting of two identical diffusers are producible for wide interferometer gaps because the order value is kept close to zero for any diffuser separation. The channelled spectrum is used to determine the dispersion of a glass plate whose refractive index is accurately determined to 3 parts in 103for wavelengths having an error of 0·13 nm. The application of the speckle interferometer as a refractometer is comparable with the method of minimum deviation in prisms since both refractometers deduce the refractivity from angle measurements.