On tonometers

Tonometry is the measurement of the internal pressure of the eyeball. It's an important diagnostic tool. High Intraocular pressures (IOP) are damaging to someone's sight.
Here is an overview of the different tonometers used to measure IOP:

1. Thumbs : (Don't panic, we're not going down that road again!) By gently palpating the orbs (ooh missus!) the experienced Ophthalmologist can detect differing IOPs between the two eyes.
2. Schiötz : A purely mechanical device, now consigned to history with the sextant, the astrolabe and Windows 95. It doesn't require electricity, so it might be useful if you're holding a clinic in a tent, miles from civilisation. Normal location is in an eye hospital cupboard with other arcane items of long unused pieces of equipment (such as the big disc with concentric black and white rings on it.)
3. Pneumatonometer : Only ever found at the opticians. It uses a small measured blast of air to obtain the reading, thus there is no contact with the cornea, and no risk of infection or abrasion. Of course, it means that patients who visit us screw their eyes up in anticipation of that puff of air!
4. Goldmann : Applanation tonometer attached to a slit lamp. Uses a disposable plastic split prism, that, when pushed against a flourescein-stained eye, under a blue light, displays two offset green semi-circles. You turn the dial to move them into position, then read the pressure reading off the dial.
5. Perkins : Like the Goldmann, only portable. Now superseded by :-
6. Tonopen : It's new. It's totally portable - you can use it on all those patients who you can't get to the slit lamp. You hold it in your hand like a dart, and gently tap the patients eyeball, each time getting a little feedback click until it goes beep, and there's the reading. To prevent cross-infection, it comes with a supply of little rubber sheaths which you unroll over the tip of your magic wand ... okay okay, if they look like condoms, and they feel like condoms, then they are condoms - if you're a hobbit!