The wavefront from a celestial source is believed to arrive at the outer fringe of the Earth’s atmosphere as a truly plane wave of effectively infinite extent. After propagating through our refractively non-uniform atmosphere this wavefront becomes distorted in shape and exhibits a nonuniform amplitude distribution. At present there is little that can be done to remedy the effects of the atmosphere on wave propagation. However when the wave has been received by the entrance aperture of the observing telescope we should try to design this optical system, including its air-path sections, so as to minimize further distortion of the wavefront. ‘Seeing’ effects within an instrument which concern us in this paper are attributable to air of nonuniform temperature (and hence refractive index) in the sections between the optical components.