The focus of the collimating lens in the optical train of a Fizeau interferometer is adjusted to change the power of the test beam illuminating the transmission sphere. As a result, the rays can be made sufficiently perpendicular to the reference surface to eliminate the chromatic focus shift and non-common path errors produced by a light source of wavelength different from the design wavelength of the transmission sphere. By making the position of the collimating lens relative to the beam expander adjustable along the optical axis over some small range, illumination sources of various wavelength can be used in the same interferometer.