A recording tonometer.

THE measurement of the resistance to outflow of the aqueous humour by the procedure of tonography has recently aroused interest in the continuous recording of the intra-ocular pressure (IOP). There are, in addition, numerous other physiological and clinical investigations which could be performed if it were possible to obtain a continuous and accurate record of the IOP in human subjects. Such recordings have been obtained using the Muller electronic tonometer connected to a suitable amplifier and recording galvanometer. This arrangement, however, suffers from several disadvantages. First, the tonometer must be held continuously on the patient's eye by an operator, and this limits the time during which the record can be continuously taken. An improvement may be effected by using a mechanical holder, such as that designed by Merte (1957), but some supervision would still be required. Secondly, the fact that it is impossible for the patient to move his head makes many experimental procedures impossible. Thirdly, the Muller instrument, being basically a Schi6tz tonometer, suffers from the defects of this instrument, principally that its application to the eye raises its IOP to about double its original value (Friedenwald, 1954), the extent of the rise varying from eye to eye in a not readily determinable manner. The instrument described in this paper is essentially a research tool and is too complicated for widespread use as a routine clinical instrument. It was designed to be mounted on the patient's forehead and to maintain its position on the eye without attention and regardless of the position of the patient's head. In addition, it seems to be intrinsically more accurate than the Schiotz tonometer. The theoretical principle on which it operates and its practical construction are first explained, and further sections describe its method of use and its performance. Only the results of investigations into normal eyes are reported in this paper; it is intended to publish studies on glaucomatous patients later. Finally, an account of the calibration of the tonometer on human cadavers is given, from which the value of the normal IOP is derived.