Standardized x-tonometers.

STANDARDIZED Schi6tz tonometers of the original (weighted) type have been on sale in the United States for more than 10 years, and a considerable amount of information has been collected which bears out the acceptability of the current specification. This knowledge was assembled in the First Decennial Report of the Committee on Standardization of Tonometers (1954). Following the American lead, standardized tonometers have been introduced into the United Kingdom, and the work of the W. H. Ross Foundation (Scotland) in this field has already been described (Jackson, 1953, 1955). The specification for the weighted model of the Schiotz tonometer is now internationally accepted, as is the new conversion of scale readings into mm. Hg (Schi6tz) intra-ocular pressure. The instrument manufacturers, both in the U.K. and U.S.A., now submit their products to independent testing stations 'for examination and, if satisfactory, for certification as standard tonometers. These measures have at least introduced a considerable degree of uniformity between different tonometers, but it still has to be remembered that this type of tonometer is not without its own sources of instrumental error, which have been assessed by Friedenwald (1937), and that we must add to these the variables introduced by different observers, by the degree of relaxation of the patient, and his position, and by the time of day. I believe that the tendency to read a Schi6tz tonometer to a degree of accuracy closer than the whole scale division is a mistaken one, and that the current calibration charts, which suggest a value for the various scale readings accurate to fractions of a millimetre of mercury, imply a standard of accuracy which is entirely misleading. Some years after the introduction of his original tonometer, Schiotz (1927) described his tonometer with a convex plunger. This is what we know as the X-tonometer, and is without the additional weights which were used in the original model to adjust the instrument to cover the various ranges of intra-ocular pressure. The different shape of the plunger-end enables a wide range of intra-ocular pressures to be registered without the need to alter the weight acting on the plunger. In the United States, the X-tonometer attracts very little interest. In the United Kingdom and in Scandinavia, on the other hand, the weightless model is very popular, and its sales greatly outnumber those of the weighted model. From a study of Schiotz's original description of the weightless tonometer, together with the work of the American Committee, a specification has been