October 6th, letters section:

SIR - Those about to invest in iris-scanning security technology will be disappointed to learn of recent developments in the treatment of glaucoma ("Watching you ", September 22nd). Prostaglandin analogues are rapidly gaining popularity in the treatment of this blinding eye condition that affects 1% of the population. An innocuous side-effect of this drug is to cause a change in both iris colour (a darkening) and morphology. This change in susceptible people, usually Europeans, occurs over one to two years. Apart from rendering iris scanning potentially useless for these people, unscrupulous types without glaucoma may be tempted to use the drugs to "change" identity.

Simon Longstaff Consultant ophthalmic surgeon Sheffield, South Yorkshire

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