The adaption of the schlieren method for the purpose of observing flow in water is a recent development at the Garfield Thomas Water Tunnel. Flow is shown over an opening and over a semicylinder for a range of Reynolds numbers of 2.5 × 104 to 25 × 104. The relative effects of temperature and pressure as they affect the index of refraction are treated. The relative persistency of a temperature gradient in air and water is considered, and the relative effects in air and water of a chosen density gradient are calculated.