Two new methods of measuring the internal diameters of glass and other transparent tubes are described. The first consists in immersing the tube to be tested in a suitable liquid and varying the wave length of the illuminating beam until equality of refractive index is obtained. The system being now homogeneous, the diameter can be measured directly. Information is given with regard to the most suitable liquids to employ and the paper contains a number of practical hints which will facilitate the carrying out of the test. The second method consists in taking an X-ray shadow photograph of the tube and measuring the dimensions of the image formed. Between the two methods it is possible to measure accurately the internal diameter of practically any type of transparent tube, whether sealed or open, no matter what the outside shape of the tube may be.