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The fact that repeated tonometry is associated with an apparent fall in intraocular pressure was first noted by Stocker 1958 and Goldmann 1958. Since then it has been strongly confirmed (Moses 1961, Bechrakis 1966 and there is full agreement on the following points:1. When the measurements are repeated every minute there is a fall of 3-4 mm Hg in 5 minutes and about 5 mm Hg in 12 minutes. According to Bechrakis the fall is exponential. On the contralateral eye there is a sympathetic pressure fall.2. The decline in intraocular pressure is observed both on normal subjects and patients with glaucoma.3. The effect is found both on seated patients and on supine patients, both with applanation tonometry and with Schiotz tonometers. On the other hand, there are differences in opinion as to the effect of accommodation (Armaly, Bechrakis) and anaesthetics.The mechanism of this phenomenon is still obscure, and there are a few elementary questions to he answered before it can be tackled.1. Is the effect due to an interference from the measuring instrument or is the mere sitting at the instrument enough to elicit it? In the former case, how soon is the presure restored?2. Is the weight of the instrument resting on the cornea of any importance to the magnitude of the effect? Is it possible to measure the intraocular pressure at all without provoking the effect?

Repeated applanation tonometry every minute in normal subjects gave a direct pressure reduction of about 3 mmHg in five minutes and a lower consensual pressure reduction. Sham measurements had no corresponding effect, nor was the initial IOP reduced after sitting at the tonometer for five minutes before tonometry. The spacing of the measurements and the length of time the applanation body was left on the cornea influenced the decrease curve.K e y words: applanation tonometry -sham measurements -consensual pressure reduction.Since Stocker (1 958) observed a pressure reduction following repeated tonometry, this phenomenon has been studied by several authors (Moses 1961, Bechrakis 1966, and others). Several aspects of their results have been confirmed i n experiments by the present author. T h e direct pressure reduction amounts to 3-4 m m H g in 5 minutes and follows a curve which no doubt is well described by a n exponential function. There is, however, a n important point on which the findings made in our laboratory a r e not in accordance with those of Moses and Bechrakis. These authors claim that sham measurements are as efficient as true measurements i n reducing the pressure. This is difficult to reconcile with ob-

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