He was pointing to a letter "r." Under the glass I noticed that there was a break in the little curl at the top.
"Now if you find such a break in the same letter in another piece of typewriting, what would you think?"
"That they were from the same machine," I replied.
"Not so fast," he cautioned. "True, it might raise a presumption that it was from the same machine. But the laws of chance would be against your enthusiasm, Walter."
"Of course," I admitted on second thought.
"It's just like the finger-print theory. There must be a sort of summation of individual characteristics. Now here's a broken 'l' and there is an 'a' that is twisted. Now, if the same defects are found in another piece of writing, that makes the presumption all the stronger, and when you have massed together a number of such characteristics it raises the presumption to a mathematical certainty, does it not?"
I nodded and he went on. "The faces of many letters inevitably become broken, worn, or battered. Not only does that tend to identify a particular machine, but it is sometimes possible, if you have certain admitted standard specimens of writing covering a long period, to tell just when a disputed writing was made. There are two steps in such an inquiry, the first the determination of the fact that a document was written on a certain particular kind of machine and the second that it was written on a certain individual machine of that make. I have here specimens of the writing of all the leading machines. It is easy to pick out the make used, say in the 'Outcast' letter. Moreover, as I said when I first saw that letter, it is in the regular pica type. So are they all, but as ninety-five per cent, use the pica style that in itself proved nothing."
"What is that bit of ruled glass?" asked Clare, bending over the letters in deep interest.
"In ordinary typewriting," replied Craig, "each letter occupies an imaginary square, ten to the inch horizontally and six to the inch vertically. Typewriting letters are in line both ways. This ruled glass plate is an alinement test plate for detecting defects in alinement. I have also here another glass plate in which the lines diverge each at a very slightly different angle—a typewriting protractor for measuring the slant of divergence of various letters that have become twisted, so to speak.
"When it is in perfect alinement the letter occupies the middle of each square and when out of alinement it may be in any of the four corners, or either side of the middle position or at the top or bottom above or below the middle. That, you see, makes nine positions in all—or eight possible divergences from normal in this particular alone."