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March 27, 2008

New York, N.Y.

From its earliest conception, I knew that my forthcoming book
*The Annotated Turing*
would contain the complete text of Alan Turing's 36-page paper (and 3-page correction) "On Computable Numbers, with an Application to the Entscheidungsproblem," originally published in the *Proceedings of the London Mathematical Society* in 1936 and 1937.

There are basically two ways you can reproduce a 1936 paper in a modern book: You can scan the original paper and essentially treat it like a bitmap, or you can entirely re-set the paper as if it were any other text.

The first approach is used in *The Collected Works of A. M. Turing* and Martin Davis's 1965 book *The Undecidable: Basic Papers on Undecidable Propositions, Unsolvable Problems and Computable Functions*. In theory I like this approach because the reader is actually encountering the original paper as it looked 70 years ago. However, depending on the condition of the original source, the result can look a bit gritty, and you can even get some broken characters. Perhaps the best scan I've seen of Turing's paper is
this one and it's certainly not perfect.

The second approach (re-setting the paper) is used in B. Jack Copeland's book *The Essential Turing* and also in
this online PDF version. The paper becomes very readable (except in the online PDF version) but it doesn't look quite "real." It doesn't stand out as an object foreign to the modern parts of the book. The possibility also exists that new errors will be introduced when it's re-set.

For *The Annotated Turing* I had another major consideration. The book contains not only Turing's paper but my commentary and explanations interspersed with the paper. Turing made use of a German gothic font and an unusual script font in his paper. To minimize confusion, I would need to use those same fonts in my commentary and annotations, and I suspected it would be quite difficult to match the fonts exactly.

For those reasons, I wanted Turing's paper to be re-set in my book, but I also wanted it to look as close as possible to the original published version. I struggled with this problem in Microsoft Word (as I have described) so I was well familiar with the difficulties involved. Besides the multiple fonts, the paper contains a lot of strange tables and mathematical formulas with subscripted subscripts.

I discussed these issues at length with the book's Development Editor at Wiley, and he agreed that the paper should be re-set. I told him "I want to be able to proof-read the re-set paper by printing it on transparency film and putting it on top of the original version." He was amused by this concept, and of course neither of us expected the scheme to work. Small differences in type size and line spacing make an exact match impossible.

But I had to try. When I recently received a PDF of the re-set paper to review, I printed it on transparency film, and positioned each page of film on top of a normal printout of the original paper. I discovered that I could proof-read the pages not by an exact positioning, but so that lines of text alternated, like this:

That's a 200-DPI scan with the re-set version printed on transparency film oriented slightly above a printout of the original paper. By sliding the transparency around on the original, I was able to compare the two versions quite easily.

As you can see, the two fonts aren't *precisely* the same, but they're both variants of Century Schoolbook. I think that people familiar with the paper from *The Collected Works* or Davis's *The Undecidable* will pause a bit when encountering the paper in my book and ask "Is it real? Or is it re-set?" and not be able to decide for sure right away.

That's precisely the effect I wanted.

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(c) Copyright Charles Petzold

www.charlespetzold.com

Comments:That seems to be a pretty effective way of proofing - one of the many reasons that I think that we'll never really be able to move to a paperless society (not any time soon, anyway!)

What I did find strange however, was that you wanted to use the same typeface for commentary and annotation to minimise confusion. I would have thought that this would cause the opposite effect - surely having these elements in a different font would aid the user in separating them from the original text?

— Carl, Wed, 2 Apr 2008 06:25:38 -0400 (EDT)

Sorry for any confusion: We're using

differenttypefaces for Turing's paper and my commentary. Also, Turing's paper will be printed with a light gray background. — CharlesWe used to call this technique "visual diff" for quickly analyzing student submissions of programming projects to determine if there was any significant amount of copying between them.

What program did your publisher use for the typesetting? Based on the fonts and some eye-ball analysis, my guess would be LaTeX. I typeset the same passages with Computer Modern and produced almost identical output as the bottom version of the example text, modulo some minor spacing and line width issues, which your publisher seems to have admirably handled.

— T. Hudson, Sun, 6 Apr 2008 12:30:55 -0400 (EDT)

T. Hudson is correct, the program used to typeset The Annotated Turing was LaTeX.

Chris Webb

Executive Editor

John Wiley & Sons

— Chris Webb, Wed, 9 Apr 2008 13:02:47 -0400 (EDT)

I think a paperless solution would have been easy here. Photo editors can handle transparency, composition, etc.

Interesting that the modern version of the italic k dropped a curlicue, while the modern version of italic n and u added some.

— The pudding (the one where the proof was), Mon, 21 Apr 2008 23:09:49 -0400 (EDT)