How to use this book

Reading order

The chapters are written to be read in order, from Part I to Part IV, and each ends with links to the previous and next chapter. Each chapter closes with a short list of its key evidence, linked to the sections that hold it. If you want the core of the story first, Chapter 4, Step by step, follows a light-sensitive patch all the way to a camera eye.

The Atlas of Eyes and Claims and responses are reference sections. Each page in them stands on its own, so you can read one without the chapters, or send a single page to someone else. Both link to the chapters that set out the fuller evidence.

The book is published a chapter at a time, as each is checked. The contents list shows the whole book from the start; chapters not yet published are greyed out and marked “coming”, and the previous and next links skip them.

Notes and sources

Every factual sentence ends with a small number. It leads to a note naming the source: a paper with its DOI, or a book with its printed page. On a wide screen the note sits in the margin beside its paragraph. On a narrower screen it appears beneath the paragraph, and on a phone you tap the number to open it.

Note numbers start again at 1 on each page. The Bibliography gathers every source from every page into one list and shows which pages cite each one.

The wording tells you how settled a finding is. A plain statement is established or rests on strong evidence. Phrases such as “the leading hypothesis is”, “most researchers think” and “this is disputed” mark questions that are still open, and where researchers disagree, the page says so.

Figures

Figures are numbered by chapter, so Figure 4.2 is the second figure in Chapter 4; on claim pages they are numbered from 1. Each has a caption saying what it shows and a credit line naming who made it, what it was drawn from and its licence. The Figure and media credits page lists every image in the book.

Words and search

Technical terms are explained where they first appear. Search covers every published page, and the Atlas index can be filtered by eye type and by animal group.

Linking and citing

Page addresses are permanent, and chapter and section numbers never change once published. “Half an Eye, section 4.3” will always point to the same section.

Each page has a “Cite this page” box with a ready-made citation that includes the date the page was last revised. Every page shows when it was first published and last revised, and the Changelog records each change, so you can see whether a page has changed since you cited it.

If you need one link to share, a claim page or an Atlas entry is written to be read on its own, with its sources attached.

Reusing the text

The text is free to copy, share and adapt under the CC BY 4.0 licence, as long as you credit Half an Eye. Photographs and some figures carry their own licences. The About page gives the details, and explains how the book was researched and checked.