The centrepiece of the show will be Leonardo da Vinci's famous Codex Leicester, one of the artist's most significant scientific manuscripts. Focusing on the theme of water, it was composed in Milan between 1508 and 1510 and is written in his distinctive mirror script.

It took the Chester Beatty four years to arrange the borrowing of this manuscript, which travels abroad just once a year. It is the only da Vinci notebook still in private hands, having been bought by Bill Gates in 1994 for a staggering $30.8m (although this is small change for the Microsoft chairman, whose wealth is estimated at $56bn).

"I remember going home one night and telling my wife Melinda that I was going to buy a notebook, " Gates recalled. "She didn't think that was a very big deal. I said, no, this is a pretty special notebook, this is the Codex Leicester, one of the notebooks of Leonardo da Vinci. And I personally have always been amazed by him because he personally worked out science on his own and he understood things that no other scientist of that time did."

No doubt that last comment hints at a certain connection Gates sees between himself and Leonardo da Vinci, particularly as online digital versions of da Vinci's Codex Leicester and 'Codex Arundel' were unveiled by the British Library in January . . . as part of the product launch of the new Windows Vista operating system (see www. bl. uk).

The legacy of Leonardo da Vinci (14521519) . . . painter, engineer, philosopher, architect, inventor and all-round Renaissance man . . . has been hijacked in many ways over the years, not least by that book, Dan Brown's The Da Vinci Code. This exhibition, in contrast, aims to celebrate da Vinci's achievements while also putting his scientific work in context. "There is this kind of myth about Leonardo in the western world that he was everything . . . and he was a little bit of everything. But people think he invented almost everything that was ever going to be invented. That's not strictly speaking true; he relied a lot on contemporaries who themselves were inventing things and he did a lot of things that looked like inventions in his manuscripts but were really only experiments in his head for testing out his scientific hypotheses, " says Dr Michael Ryan, director of the Chester Beatty Library.

"He has of course been hijacked by all sorts of fanciful theorists. The Da Vinci Code is just one example of people assuming Leonardo belonged to some ancient brotherhood that understood the secrets of the universe. The truth is an awful lot less than thatf People believe all sorts of things about him but he was a man of his times . . . although a very, very brilliant man."

The Codex Leicester will be exhibited alongside scientific Islamic manuscripts from the Chester Beatty collection, some dating as far back as the ninth century, as well as books and manuscripts from da Vinci's time borrowed from the Cambridge, Trinity and Edward Worth libraries. These works are rare jewels in themselves and should help put da Vinci's manuscript within a cultural and scientific context.

About 6,000 pages of Leonardo da Vinci's notebooks survive today, although this is believed to be only about a quarter of his writings. The Codex Leicester comprises 18 loose double sheets, featuring about 300 drawings and diagrams that illustrate his theories and experiments. The manuscript was acquired in 1717 by the first earl of Leicester (hence the name) and remained in his family until 1980, when it was bought by American industrialist Armand Hammer, who renamed it the Codex Hammer. When it was bought by Gates, the earlier title was restored.

The Codex Leicester is a compilation of thoughts, observations and . . . sometimes . . .

flights of fancy. While the theme of water pervades, da Vinci's musings reveal the diversity of his interests, covering subjects from astronomy and meteorology to geology and the environment. He observes wave motion, currents, drops and bubbles, while also addressing practical topics such designing canals and bridges, controlling water flows, preventing silting and draining marshes. In one section, he even notes that he has devised a way of staying under water for long periods of time but, being wary that the manuscript could end up in the wrong hands, he refuses to write down the details of this invention.

Indeed, da Vinci offered his services as a military and hydraulic engineer to several patrons and, in 1,500, helped the Venetians increase their lagoon defences against the possibility of Turkish invasion. While in Florence, he and Machiavelli devised a plan to divert the course of the river Arno, thus depriving Pisa of water and access to food supplies. Luckily for the Pisans, this never materialised.

As Ryan notes in the exhibition catalogue, da Vinci's lack of academic training helped him develop a keen eye for "reading in the book of nature". His powers of observation . . . both artistic and scientific . . . along with his shrewd mind and strong imagination helped him develop new theories about the world around him.

In the Codex Leicester, for example, he was the first to suggest that the moon not only reflected the light of the sun but also the light of the earth. However, he also developed a complicated hypothesis that the moon reflected light because it was covered in choppy water. Likewise, he denied the importance of the moon in the ebb and flow of the earth's tides.

In the manuscript, da Vinci also attempted to refute the widely held belief that the discovery of seashell fossils on mountains was proof that the biblical flood took place. While he did not dispute the flood itself, he suggested an alternative reason for their presence . . . that the land had been pushed upwards over time by geological forces. In this, he was in effect prefiguring 19th-century scientific geology. But related to this idea was another theory, which crops up repeatedly in the text and which he clung to tenaciously:

that the Earth acted like a living organism similar to the body, with veins of water pulsing through it as blood runs through our veins.

While the notebook centres on empirical observations and theories, it also throws light on da Vinci's paintings. His theories on the effect of atmosphere on colour can be seen in the background of the 'Annunciation', which is enveloped in a blue, hazy mist, while in the 'Virgin and Child with St Anne', the figures seem to echo the shape of the landscape, as if drawing a parallel between the body and the earth, the microcosm and the macrocosm. Even the 'Mona Lisa', with her enigmatic expression, is compared by Philip Cottrell in the exhibition catalogue with the fluctuations of the moon.

The Codex Leicester, perhaps intended as a draft for future publication, is essentially da Vinci 'thinking on paper'. "It's very revealing about his scientific thinking, " says Ryan.

"It was written late in his life and it's a distillation of his ideas, so there are lots of lists of hypotheses and contents of books he never got around to writing. It's quite technical but it's remarkable and there are observations in it that show that he was getting close to some of the ideas of fundamental science that weren't really made explicit until a couple of centuries later."

However, the manuscript also demonstrates da Vinci's unfortunate inability to finish projects . . . his writings never made it into book form, while many painting commissions were never completed. With an insatiable curiosity for the world around him, he was frequently diverted from the task at hand. Even with 'The Last Supper' he insisted on experimenting with a new fresco technique, with the result that this Renaissance masterpiece began to deteriorate almost as soon as it was finished. As he wrote repeatedly in his manuscripts: "Tell me if anything was ever done."

But even taking this into account, the Codex Leicester goes some way towards deciphering the real da Vinci code . . . the genius that was Leonardo himself.

'Leonardo: the Codex Leicester . . . Reflections on Water and the Moon' runs from 13 June to 12 August at the Chester Beatty Library. Admission is free but booking is essential. For more details and to book go to www. cbl. ie