ves were almost never cataloged alphabetically because so many of the authors were unknown。 Titles didn't work either because many historical documents were untitled letters or parchment fragments。 Most cataloging was done chronologically。 Disconcertingly； however； this arrangement did not appear to be chronological。
Langdon felt precious time already slipping away。 〃Looks like the Vatican has its own system。〃
〃What a surprise。〃
He examined the labels again。 The documents spanned centuries； but all the keywords； he realized； were interrelated。 〃I think it's a thematic classification。〃
〃Thematic？〃 Vittoria said； sounding like a disapproving scientist。 〃Sounds inefficient。〃
Actually 。 。 。 Langdon thought； considering it more closely。 This may be the shrewdest cataloging I've ever seen。 He had always urged his students to understand the overall tones and motifs of an artistic period rather than getting lost in the minutia of dates and specific works。 The Vatican Archives； it seemed； were cataloged on a similar philosophy。 Broad strokes 。 。 。
〃Everything in this vault；〃 Langdon said； feeling more confident now； 〃centuries of material； has to do with the Crusades。 That's this vault's theme。〃 It was all here； he realized。 Historical accounts； letters； artwork； socio…political data； modern analyses。 All in one place 。 。 。 encouraging a deeper understanding of a topic。 Brilliant。
Vittoria frowned。 〃But data can relate to multiple themes simultaneously。〃
〃Which is why they cross…reference with proxy markers。〃 Langdon pointed through the glass to the colorful plastic tabs inserted among the documents。 〃Those indicate secondary documents located elsewhere with their primary themes。〃
〃Sure；〃 she said； apparently letting it go。 She put her hands on her hips and surveyed the enormous space。 Then she looked at Langdon。 〃So； Professor； what's the name of this Galileo thing we're looking for？〃
Langdon couldn't help but smile。 He still couldn't fathom that he was standing in this room。 It's in here； he thought。 Somewhere in the dark； it's waiting。
〃Follow me；〃 Langdon said。 He started briskly down the first aisle； examining the indicator tabs of each vault。 〃Remember how I told you about the Path of Illumination？ How the Illuminati recruited new members using an elaborate test？〃
〃The treasure hunt；〃 Vittoria said； following closely。
〃The challenge the Illuminati had was that after they placed the markers； they needed some way to tell the scientific munity the path existed。〃
〃Logical；〃 Vittoria said。 〃Otherwise nobody would know to look for it。〃
〃Yes； and even if they knew the path existed； scientists would have no way of knowing where the path began。 Rome is huge。〃
〃Okay。〃
Langdon proceeded down the next aisle； scanning the tabs as he talked。 〃About fifteen years ago； some historians at the Sorbonne and I uncovered a series of Illuminati letters filled with references to the segno。〃
〃The sign。 The announcement about the path and where it began。〃
〃Yes。 And since then； plenty of Illuminati academics； myself included； have uncovered other references to the segno。 It is accepted theory now that the clue exists and that Galileo mass distributed it to the scientific munity without the Vatican ever knowing。〃
〃How？〃
〃We're not sure； but most likely printed publications。 He published many books and newsletters over the years。〃
〃That the Vatican no doubt saw。 Sounds dangerous。〃
〃True。 Nonetheless the segno was distributed。〃
〃But nobody has ever actually found it？〃
〃No。 Oddly though； wherever allusions to the segno appear…Masonic diaries； ancient scientific journals； Illuminati letters…it is often referred to by a number。〃
〃666？〃
Langdon smiled。 〃Actually it's 503。〃
〃Meaning？〃
〃None of us could ever figure it out。 I became fascinated with 503； trying everything to find meaning in the number…numerology； map references； latitudes。〃 Langdon reached the end of the aisle； turned the corner； and hurried to scan the next row of tabs as he spoke。 〃For many years the only clue seemed to be that 503 began with the number five 。 。 。 one of the sacred Illuminati digits。〃 He paused。
〃Something tells me you recently figured it out； and that's why we're here。〃
〃Correct；〃 Langdon said； allowing himself a rare moment of pride in his work。 〃Are you familiar with a book by Galileo called Diаlogo？〃
〃Of course。 Famous among scientists as the ultimate scientific sellout。〃
Sellout wasn't quite the word Langdon would have used； but he knew what Vittoria meant。 In the early 1630s； Galileo had wanted to publish a book endorsing the Copernican heliocentric model of the solar system； but the Vatican would not permit the book's release unless Galileo included equally persuasive evidence for the church's geocentric model…a model Galileo knew to be dead wrong。 Galileo had no choice but to acquiesce to the church's demands and publish a book giving equal time to both the accurate and inaccurate models。
〃As you probably know；〃 Langdon said； 〃despite Galileo's promise； Diаlogo was still seen as heretical； and the Vatican placed him under house arrest。〃
〃No good deed goes unpunished。〃
Langdon smiled。 〃So true。 And yet Galileo was persistent。 While under house arrest； he secretly wrote a lesser…known manuscript that scholars often confuse with Diаlogo。 That book is called Discorsi。〃
Vittoria nodded。 〃I've heard of it。 Discourses on the Tides。〃
Langdon stopped short； amazed she had heard of the obscure publication about planetary motion and its effect on the tides。
〃Hey；〃 she said； 〃you're talking to an Italian marine physicist whose father worshiped Galileo。〃
Langdon laughed。 Discorsi however was not what they were looking for。 Langdon explained that Discorsi had not been Galileo's only work while under house arrest。 Historians believed he had also written an obscure booklet called Diagramma。
〃Diagramma della Veritа；〃 Langdon said。 〃Diagram of Truth。〃
〃Never heard of it。〃
〃I'm not surprised。 Diagramma was Galileo's most secretive work…supposedly some sort of treatise on scientific facts he held to be true but was not allowed to share。 Like some of Galileo's previous manuscripts； Diagramma was smuggled out of Rome by a friend and quietly published in Holland。 The booklet became wildly popular in the European scientific underground。 Then the Vatican caught wind of it and went on a book…burning campaign。〃
Vittoria now looked intrigued。 〃And you think Diagramma contained the clue？ The segno。 The information about the Path of Illumination。〃
〃Diagramma is how Galileo got the word out。 That I'm sure of。〃 Langdon entered the third row of vaults and continued surveying the indicator tabs。 〃Archivists have been looking for a copy of Diagramma for years。 But between the Vatican burnings and the booklet's low permanence rating； the booklet has disappeared off the face of the earth。〃
〃Permanence rating？〃
〃Durability。 Archivists rate documents one through ten for their structural integrity。 Diagramma was printed on sedge papyrus。 It's like tissue paper。 Life span of no more than a century。〃
〃Why not something stronger？〃
〃Galileo's behest。 To protect his followers。 This way any scientists caught with a copy could simply drop it in water and the booklet would dissolve。 It was great for destruction of evidence； but terrible for archivists。 It is believed that only one copy of Diagramma survived beyond the eighteenth century。〃
〃One？〃 Vittoria looked momentarily starstruck as she glanced around the room。 〃And it's here？〃
〃Confiscated from the Netherlands by the Vatican shortly after Galileo's death。 I've been petitioning to see it for years now。 Ever since I realized what was in it。〃
As if reading Langdon's mind； Vittoria moved across the aisle and began scanning the adjacent bay of vaults； doubling their pace。
〃Thanks；〃 he said。 〃Look for reference tabs that have anything to do with Galileo； science； scientists。 You'll know it when you see it。〃
〃Okay； but you still haven't told me how you figured out Diagramma contained the clue。 It had something to do with the number you kept seeing in Illuminati letters？ 503？〃
Langdon smiled。 〃Yes。