Be on your guard for the curse of the infamous slide projector
In America you can make a career out of lecturing on presentation. I remember listening enthralled to one man who explained that it was almost impossible to trust any arrangement of slides in a carousel.
'They'll jump around in the box, ' he said. 'Even if you put a book on top of them overnight, by morning one of them will have turned itself upside down and another will have gotten lost.'
Whether that man is now trading on PowerPoint disasters I do not know, but there is no doubt that, despite digitisation, putting your images in order is still the low blood sugar part of lecturing, and for those who still use slides - because they have millions of them - the nightmare never ends.
Let us begin with the slide cabinet: sometimes homemade - in which case it is too small;
sometimes professionally manufactured - in which case it is even smaller. We open it in trepidation, always far too late. We spent too much time writing the lecture and now too little time is left for this.
Still the slides seem quiet enough, backlit in their rows of metal holders, but do not be fooled. Their coefficients of friction have been specially designed to trap the unwary by spilling a whole rack if you make a hasty movement. Better to put down your coffee first and use both hands. Whoops!
Having tidied up that mess, we carefully slide the first rack out of the park position so that we can begin our search. At first the slides seem foreign and profoundly irrelevant. Every one of them is worth a thousand words, of course, but they seem to be the wrong words. After an hour of careful picking out, we have only illuminated the foothills of the lecture - if the audience can supply the links that are still missing.
What, for instance, is the connection between Daumier's 'Two Nymphs pursued by a Satyr' and a snapshot of two Teletubbies with their heads off?
For some reason, in a previous life, we ordained that these images should be juxtaposed but now we cannot remember why.
The same is true of other surreal pairings. A man made of polystyrene beads and a small child watching Winnie the Pooh on TV, for instance; or a family of crash-test dummies staring at Asimo, the robot who can walk downstairs.
In desperation we resort to the bulging rack labelled 'Buildings' where, in less than half an hour, Number One Poultry has been picked out to indicate the quirky nature of the City of London compared with the buttondown towers of Canary Wharf.
This is progress, but at a price. It is nearly midnight by the time the rack that has to be decanted into carousel trays is full.
This is the tricky bit. There are 100 slides in the rack and we want to show them on two projectors side by side. That means that the 50 slides intended for the left-hand projector must be taken off the rack and placed in the left-hand carousel, so as to be synchronised with the 50 slides intended for the righthand projector.
Somehow, late at night, this simple task gradually becomes impossible. Twice we end up with 47 slides in one carousel and 53 in the other and, because we have fasttracked the whole process by not writing down the names of the slides, we have no way to correct the error except by putting all the slides - left, right, left, right - back into the rack and start all over again.
In the end, we decide that the only option is to go to bed leaving them hanging in the rack. At least that way they will not have turned upside down or been lost.