Still, as they say down the Walworth Road, principles are there to be broken. If any recent CD-ROM is worth making an exception for, it's Thames & Hudson's Escher Interactive: Exploring The Art of the Infinite (pounds 49.95). In the Sixties and Seventies, M C Escher mesmerised a generation which depended upon hallucinogens to animate his geometrical paradoxes. Nowadays you can let the pixels shoulder the burden instead of your synapses, and watch the ants going round the Mobius strip for as long as your attention span permits. This CD-ROM sets a new benchmark for digitally-simulated hallucinations. The best is the effect which mimics passing a spherical lens over an Escher design, as the artist himself was wont to do.
There is also a puzzle, posing impossible challenges based on impossible shapes, which will convince all but a handful of visuo-spatial geniuses that they are a lower form of cognitive life. Marginally easier sport can be had with a game which tests the sense of perspective; more or less to destruction.
For respite, specimens of Escher's tessellating creatures can be re-coloured and reshaped in the Tessellation Workshop. You can create your own designs from scratch; these can be printed out, being free from copyright restrictions. Or you can click on the T-shirt icon to save a file which you can then place on a floppy disk, and hand to a T-shirt printer.
Images created in the Tessellation Workshop are also transferred automatically to the morphing module, where you can instruct the program to turn them into mythical Escher beasts. Escher, of course, performed these transformations by hand, long before morphing became a standard animation routine. As one toys with the disk's manipulative engines, the insidious feeling develops that Escher's art was waiting upon digital technology for its consummation. Which means that Escher Interactive is highly persuasive as a demonstration of the CD-ROM form; but I wonder what the Great Draughtsman himself would have thought.
The real revelation is in the "gallery" section. This contains Escher's entire catalogue of graphic works, more than 400 of them, along with drawings and sketches. One may search by keyword, separating out, for example, all the designs containing images of fish. But I found the greatest pleasure in proceeding chronologically, observing the development of his technique. This revealed an exquisite portfolio of softer forms and colours, clearly ancestral to the later hard-angled intricacies of his best known work, yet very different - and far more engaging - in mood.
There remains the vexed question of the Windows-only format. John Hawkins, who runs multimedia at Thames & Hudson, is a Mac devotee, but he explains that since this disk required a formidable amount of mathematical coding, creating a Mac version would have added greatly to the cost. But for most CD-ROMs, compatibility with both formats doesn't cost much extra these days; for their publishers, there is no excuse.