A beam of parallel particles of given velocity traversing a transparent radiator gives off Cerenkov radiation at a unique angle with respect to the beam direction independently of the point of traversal. Such light can be brought to a ring focus. A Cerenkov counter has been built using this principle. Velocity definition is obtained by using a ring diaphragm on the face of a photomultiplier. The greatest velocity definition is obtained for small angle Cerenkov light; we have chosen 10 degrees. To change the velocity selection without changing the optics the index of refraction of the radiator is varied. The index of refraction may be varied by changing the pressure of a gas near its critical point. The working fluid is Fluorochemical FC-75.1 The index of refraction of FC-75 can be varied continuously from n = 1.15 to n = 1.01 at a temperature of 250°C by changing the pressure. The counter has been tested using 1.8 bev/c π mesons from the Brookhaven Cosmotron; the counting rate vs pressure of the FC-75 has a full width corresponding to less than Δ ß = 0.01.